Extra-cardiovascular pacing by an implantable cardioverter defibrillator

ABSTRACT

An extra-cardiovascular implantable cardioverter defibrillator (ICD) having a low voltage therapy module and a high voltage therapy module is configured to select, by a control module of the ICD, a pacing output configuration from at least a low-voltage pacing output configuration of the low voltage therapy module and a high-voltage pacing output configuration of the high voltage therapy module. The high voltage therapy module includes a high voltage capacitor having a first capacitance and the low voltage therapy module includes a plurality of low voltage capacitors each having up to a second capacitance that is less than the first capacitance. The ICD control module controls a respective one of the low voltage therapy module or the high voltage therapy module to deliver extra-cardiovascular pacing pulses in the selected pacing output configuration via extra-cardiovascular electrodes coupled to the ICD.

REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.16/132,564, filed Sep. 17, 2018 (published as US Patent Pub. No.2019/0015671), which is a Continuation of U.S. patent application Ser.No. 15/367,516, filed Dec. 2, 2016 (granted as U.S. Pat. No.10,080,905), which claims the benefit of U.S. Patent Application Ser.No. 62/262,499, filed provisionally on Dec. 3, 2015 (now expired), thecontent of all of which is incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The disclosure relates generally to an extra-cardiovascular implantablecardioverter defibrillator (ICD) system, device and method fordelivering cardiac pacing pulses using extra-cardiovascular electrodes.

BACKGROUND

Medical devices, such as cardiac pacemakers and ICDs, providetherapeutic electrical stimulation to a heart of a patient viaelectrodes carried by one or more medical electrical leads and/orelectrodes on a housing of the medical device. The electricalstimulation may include signals such as pacing pulses or cardioversionor defibrillation shocks. In some cases, a medical device may sensecardiac electrical signals attendant to the intrinsic or pacing-evokeddepolarizations of the heart and control delivery of stimulation signalsto the heart based on sensed cardiac electrical signals. Upon detectionof an abnormal rhythm, such as bradycardia, tachycardia or fibrillation,an appropriate electrical stimulation signal or signals may be deliveredto restore or maintain a more normal rhythm of the heart. For example,an ICD may deliver pacing pulses to the heart of the patient upondetecting bradycardia or tachycardia or deliver cardioversion ordefibrillation shocks to the heart upon detecting tachycardia orfibrillation.

SUMMARY

In general, the disclosure is directed to techniques for deliveringcardiac pacing pulses to a patient's heart by a cardiac defibrillationsystem, such as an extra-cardiovascular ICD system. An ICD operatingaccording to the techniques disclosed herein delivers cardiac pacingpulses using extra-cardiovascular electrodes carried by a medicalelectrical lead extending from the ICD. The ICD includes both a highvoltage therapy module and a low voltage therapy module and isconfigured to automatically determine a pacing output configurationusing either the high voltage therapy module or the low voltage therapymodule and a selected extra-cardiovascular pacing electrode vector. Insome examples, one or more low-voltage, pacing output configurations areavailable from the low voltage therapy module including a low-voltagepacing output configuration for delivering single-pulse pacing pulsesand/or a low-voltage pacing output configuration for deliveringcomposite pacing pulses that include two or more individual pulsesdelivered within the composite pacing pulse width to evoke a singlecardiac depolarization.

In one example, the disclosure provides an extra-cardiovascular ICDincluding a high voltage therapy module, a low voltage therapy module,and a control module. The high voltage therapy module includes a highvoltage capacitor having a first capacitance, a high voltage chargingcircuit configured to charge the high voltage capacitor, and switchingcircuitry configured to couple the high voltage capacitor acrossextra-cardiovascular electrodes coupled to the ICD. The low voltagetherapy module includes multiple low voltage capacitors each having acapacitance up to a second capacitance that is less than the firstcapacitance, a low voltage charging circuit configured to charge the lowvoltage capacitors, and switching circuitry configured to selectivelycouple the plurality of low voltage capacitors to theextra-cardiovascular electrodes. The control module is coupled to thehigh voltage therapy module and the low voltage therapy module and isconfigured to select a pacing output configuration from among at least alow-voltage pacing output configuration of the low voltage therapymodule and a high-voltage pacing output configuration of the highvoltage therapy module and control a respective one of the low voltagetherapy module or the high voltage therapy module to deliverextra-cardiovascular pacing pulses by the selected one of thelow-voltage pacing output configuration or the high-voltage pacingoutput configuration via the extra-cardiovascular electrodes coupled tothe ICD.

In another example, the disclosure provides a method performed by anextra-cardiovascular ICD having a low voltage therapy module and a highvoltage therapy module. The method includes selecting by a controlmodule of the ICD a pacing output configuration from among at least alow-voltage, pacing output configuration of the low voltage therapymodule and a high-voltage, pacing output configuration of the highvoltage therapy module. The high voltage therapy module includes a highvoltage capacitor having a first capacitance, and the low voltagetherapy module includes multiple low voltage capacitors each having upto a second capacitance that is less than the first capacitance. Themethod further includes controlling a respective one of the low voltagetherapy module or the high voltage therapy module to deliverextra-cardiovascular pacing pulses in the selected one of thelow-voltage pacing output configuration or the high-voltage pacingoutput configuration via extra-cardiovascular electrodes coupled to theICD.

In another example, the disclosure provides a non-transitory,computer-readable storage medium storing a set of instructions which,when executed by a control module of an extra-cardiovascular ICD havinga low voltage therapy module and a high voltage therapy module, causethe ICD to select a pacing output configuration from among a low-voltagepacing output configuration of the low voltage therapy module and ahigh-voltage pacing output configuration of the high voltage therapymodule. The high voltage therapy module includes a high voltagecapacitor having a first capacitance, and the low voltage therapy moduleincludes multiple low voltage capacitors each having up to a secondcapacitance that is less than the first capacitance. The ICD is furthercaused to control a respective one of the low voltage therapy module orthe high voltage therapy module to deliver extra-cardiovascular pacingpulses in the selected one of the low-voltage pacing outputconfiguration or the high-voltage pacing output configuration viaextra-cardiovascular electrodes coupled to the ICD.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a conceptual diagram of a distal portion of anextra-cardiovascular lead having an electrode configuration according toanother example.

FIG. 4 is a conceptual diagram of a distal portion of anextra-cardiovascular lead having a lead body shape according to anotherexample.

FIG. 5 is a schematic diagram of the ICD of the system of FIGS. 1A-2Caccording to one example.

FIG. 6 is a depiction of one example of a low voltage pacing pulse thatmay be generated and delivered by the low voltage (LV) therapy module ofthe ICD of FIGS. 1A-2C to pace a patient's heart usingextra-cardiovascular electrodes and a low-voltage, pacing outputconfiguration.

FIG. 7 is a depiction of one example of a high voltage pacing pulse thatmay be generated and delivered by the high voltage (HV) therapy moduleof the ICD of FIGS. 1A-2C to pace a patient's heart usingextra-cardiovascular electrodes and a high-voltage, pacing outputconfiguration.

FIG. 8 is a flow chart of one method for selecting a pacing outputconfiguration for use in delivering extra-cardiovascular cardiac pacingpulses by the ICD of FIGS. 1A-2C.

FIG. 9 is schematic diagram of a HV therapy module coupled to aprocessor and HV therapy control module of the ICD of FIGS. 1A-2Caccording to one example.

FIG. 10 is a flow chart of one method that may be performed by the ICDof FIGS. 1A-2C for enabling a high-voltage, pacing output configuration.

FIG. 11 is a conceptual diagram of a LV therapy module of the ICD ofFIGS. 1A-2C according to one example.

FIG. 12 is a flow chart of a method performed by an ICD according to oneexample.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for delivering cardiacpacing pulses using implanted, extra-cardiovascular electrodes. As usedherein, the term “extra-cardiovascular” refers to a position outside theblood vessels, heart, and pericardium surrounding the heart of apatient. Implantable electrodes carried by extra-cardiovascular leadsmay be positioned extra-thoracically (outside the ribcage and sternum)or intra-thoracically (beneath the ribcage or sternum) but generally notin intimate contact with myocardial tissue. The techniques disclosedherein provide a method for automatically configuring an ICD pacingoutput configuration using extra-cardiovascular electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of aportion of ICD system 10 implanted within patient 12. ICD system 10includes an ICD 14 connected to an extra-cardiovascular electricalstimulation and sensing lead 16. FIGS. 1A and 1B are described in thecontext of an ICD system 10 capable of providing defibrillation and/orcardioversion shocks and cardiac pacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as a housing electrode (sometimes referred to as a “can”electrode). In examples described herein, housing 15 may be used as anactive can electrode for use in delivering cardioversion/defibrillation(CV/DF) shocks or other high voltage pulses delivered using a highvoltage therapy module. In other examples, housing 15 may be availablefor use in delivering unipolar, low voltage cardiac pacing pulses inconjunction with lead-based electrodes. In other instances, the housing15 of ICD 14 may include a plurality of electrodes on an outer portionof the housing. The outer portion(s) of the housing 15 functioning as anelectrode(s) may be coated with a material, such as titanium nitride.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinan elongated lead body 18 of lead 16 and electronic components includedwithin the housing 15 of ICD 14. As will be described in further detailherein, housing 15 may house one or more processors, memories,transceivers, sensors, electrical sensing circuitry, therapy deliverycircuitry, power sources and other appropriate components.

Elongated lead body 18 includes a proximal end 27 that includes a leadconnector (not shown) configured to be connected to ICD connectorassembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead 16 includes defibrillation electrodes 24A and 24B,collectively 24, and pace/sense electrodes 28A, 28B, and 30. In somecases, defibrillation electrodes 24A and 24B may together form adefibrillation electrode in that they are configured to be activatedconcurrently. Alternatively, defibrillation electrodes 24A and 24B mayform separate defibrillation electrodes in which case each of theelectrodes 24A and 24B may be activated independently. In someinstances, defibrillation electrodes 24A and 24B are coupled toelectrically isolated conductors, and ICD 14 may include switchingmechanisms to allow electrodes 24A and 24B to be utilized as a singledefibrillation electrode (e.g., activated concurrently to form a commoncathode or anode) or as separate defibrillation electrodes, (e.g.,activated individually, one as a cathode and one as an anode oractivated one at a time, one as an anode or cathode and the otherremaining inactive with housing 15 as an active electrode).

Electrodes 24A and 24B (and in some example housing 15) are referred toas defibrillation electrodes because they are utilized, individually orcollectively, for delivering high voltage stimulation therapy (e.g.,cardioversion or defibrillation shocks).

Electrodes 24A and 24B may be elongated coil electrodes and generallyhave a relatively high surface area for delivering high voltageelectrical stimulation pulses compared to low voltage pacing and sensingelectrodes. However, electrodes 24A and 24B and housing 15 may also beutilized to provide pacing functionality, sensing functionality or bothpacing and sensing functionality in addition to or instead of highvoltage stimulation therapy. In this sense, the use of the term“defibrillation electrode” herein should not be considered as limitingthe electrodes 24A and 24B to use in only high voltagecardioversion/defibrillation therapy applications. As described herein,electrodes 24A and/or 24B may be used in a pacing electrode vector fordelivering extra-cardiovascular pacing pulses using a high-voltage,pacing output configuration.

Electrodes 28A, 28B and 30 are relatively smaller surface areaelectrodes for delivering low voltage pacing pulses and for sensingcardiac electrical signals. Electrodes 28A, 28B and 30 are referred toas pace/sense electrodes because they are generally configured for usein low voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals.In some instances, electrodes 28A, 28B, and 30 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrodes 28A and 28Bare located between defibrillation electrodes 24A and 24B and electrode30 is located distal to defibrillation electrode segment 24A. Electrodes28A and 28B are illustrated as ring electrodes, and electrode 30 isillustrated as a hemispherical tip electrode in the example of FIGS. 1Aand 1B. However, electrodes 28A, 28B, and 30 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, paddle electrodes, hemispherical electrodes,directional electrodes, segmented electrodes, or the like, and may bepositioned at any position along the distal portion 25 of lead 16.Further, electrodes 28A, 28B, and 30 may be of similar type, shape, sizeand material or may differ from each other.

Lead 16 extends subcutaneously or submuscularly over the ribcage 32medially from the connector assembly 27 of ICD 14 toward a center of thetorso of patient 12, e.g., toward xiphoid process 20 of patient 12. At alocation near xiphoid process 20, lead 16 bends or turns and extendssuperior subcutaneously or submuscularly over the ribcage and/orsternum, substantially parallel to sternum 22. Although illustrated inFIGS. 1A and 1B as being offset laterally from and extendingsubstantially parallel to sternum 22, lead 16 may be implanted at otherlocations, such as over sternum 22, offset to the right or left ofsternum 22, angled laterally from sternum 22 toward the left or theright, or the like. Alternatively, lead 16 may be placed along othersubcutaneous or submuscular paths. The path of lead 16 may depend on thelocation of ICD 14 or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24A, 24B, 28A, 28B, and 30located along the distal portion 25 of the lead body 18. Lead body 18may be tubular or cylindrical in shape. In other examples, the distalportion 25 (or all of) the elongated lead body 18 may have a flat,ribbon or paddle shape. The lead body 18 of lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors extend. However, the techniques disclosed herein are notlimited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body 18are each electrically coupled with respective defibrillation electrodes24A and 24B and pace/sense electrodes 28A, 28B, and 30. The respectiveconductors electrically couple the electrodes 24A, 24B, 28A, 28B and 30to circuitry, such as a therapy module and/or a sensing module, of ICD14 via connections in the connector assembly 17, including associatedelectrical feedthroughs crossing housing 15. The electrical conductorstransmit therapy from a therapy module within ICD 14 to one or more ofdefibrillation electrodes 24A and 24B and/or pace/sense electrodes 28A,28B, and 30 and transmit sensed electrical signals from one or more ofdefibrillation electrodes 24A and 24B and/or pace/sense electrodes 28A,28B, and 30 to the sensing module within ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. In otherexamples, lead 16 may include less than three pace/sense electrodes ormore than three pace/sense electrodes and/or a single defibrillationelectrode or more than two electrically isolated or electrically coupleddefibrillation electrodes or electrode segments. The pace/senseelectrodes 28A, 28B, and 30 may be located elsewhere along the length oflead 16, e.g., distal to defibrillation electrode 24A, proximal todefibrillation electrode 24B, and/or between electrodes 24A and 24B. Forexample, lead 16 may include a single pace/sense electrode 28 betweendefibrillation electrodes 24A and 24B and no pace/sense electrode distalto defibrillation electrode 24A or proximal to defibrillation electrode24B.

In other examples, lead 16 may include only a single pace/senseelectrode 28 between defibrillation electrodes 24A and 24B and includeanother discrete electrode(s) distal to defibrillation electrode 24Aand/or proximal to defibrillation electrode segment 24B. Various exampleconfigurations of extra-cardiovascular leads and electrodes anddimensions that may be implemented in conjunction with theextra-cardiovascular pacing techniques disclosed herein are described incommonly-assigned U.S. Pat. Publication No. 2015/0306375 (Marshall, etal.) and U.S. Pat. Publication No. 2015/0306410 (Marshall, et al.), bothof which are incorporated herein by reference in their entirety.

In still other examples, ICD system 10 of FIG. 1A and 1B may include asecond extra-cardiovascular electrical stimulation and sensing leadsimilar to lead 16. The second lead may, for example, extend laterallyto the posterior of patient 12 and include one or more electrodes thatform an electrode vector with one or more of electrodes 24A, 24B, 28A,28B, and/or 30 of lead 16 for providing pacing in accordance with thetechniques disclosed herein.

In some instances, electrodes 24A, 24B, 28A, 28B, and/or 30 of lead 16may be shaped, oriented, designed or otherwise configured to reduceextra-cardiac stimulation. For example, electrodes 24A, 24B, 28A, 28B,and/or 30 of lead 16 may be shaped, oriented, designed, partiallyinsulated or otherwise configured to focus, direct or point electrodes24A, 24B, 28A, 28B, and/or 30 toward heart 26. In this manner,electrical stimulation pulses delivered via lead 16 are directed towardheart 26 and not outward toward skeletal muscle. For example, electrodes24A, 24B, 28A, 28B, and/or 30 of lead 16 may be partially coated ormasked with a polymer (e.g., polyurethane) or another coating material(e.g., tantalum pentoxide) on one side or in different regions so as todirect the electrical energy toward heart 26 and not outward towardskeletal muscle. In the case of a ring electrode, for example, the ringelectrode may be partially coated with the polymer or other material toform a half-ring electrode, quarter-ring electrode, or otherpartial-ring electrode. When ICD 14 delivers pacing pulses viaelectrodes 24A, 24B, 28A, 28B, and/or 30, recruitment of surroundingskeletal muscle by the pacing pulses, which can cause discomfort to thepatient, may be reduced by shaping, orienting, or partially insulatingelectrodes 24 to focus or direct electrical energy toward heart 26.

ICD 14 may obtain electrical signals corresponding to electricalactivity of heart 26 via a combination of sensing vectors that includecombinations of electrodes 28A, 28B, and 30 and the housing 15 of ICD14. For example, ICD 14 may obtain cardiac electrical signals sensedusing a sensing vector between combinations of electrodes 28A, 28B, and30 with one another or obtain cardiac electrical signals using a sensingvector between any one or more of electrodes 28A, 28B, and 30 and theconductive housing 15 of ICD 14. In some instances, ICD 14 may evenobtain cardiac electrical signals using a sensing vector that includesone or both defibrillation electrodes 24A or 24B such as between eachother or in combination with one or more of electrodes 28A, 28B, and 30,and/or the housing 15.

ICD 14 analyzes the cardiac electrical signals received from one or moreof the sensing vectors to monitor for abnormal rhythms, such asbradycardia, ventricular tachycardia (VT) or ventricular fibrillation(VF). ICD 14 may analyze the heart rate and/or morphology of the cardiacelectrical signals to monitor for tachyarrhythmia in accordance with anyof a number of tachyarrhythmia detection techniques. One exampletechnique for detecting tachyarrhythmia is described in U.S. Pat. No.7,761,150 (Ghanem, et al.), incorporated by reference herein in itsentirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF). ICD 14 may deliver oneor more cardioversion or defibrillation shocks via one or both ofdefibrillation electrodes 24A and 24B and/or housing 15. ICD 14 maydeliver the cardioversion or defibrillation shocks using electrodes 24Aand 24B individually or together as a cathode (or anode) and with thehousing 15 as an anode (or cathode).

ICD 14 may generate and deliver electrical stimulation pulses other thancardioversion or defibrillation shocks, including bradycardia pacingpulses, anti-tachycardia pacing (ATP) pulses, pacing pulses duringasystole due to atrioventricular conduction block or post-shock, burstdelivery for VF induction, and/or entrainment pacing pulses before aT-shock for VF induction using a therapy vector formed from one or moreof any of a variety of electrode vectors that include one or more of theelectrodes 24A, 24B, 28A, 28B and/or 30, and/or the housing 15 of ICD14. As described below, ICD 14 may be configured to select a pacingoutput configuration using either a low voltage therapy module or a highvoltage therapy module and a pacing electrode vector selected from amongelectrodes 24A, 24B, 28A, 28B, 30 and housing 15 for delivering a pacingtherapy (e.g., ATP, asystole pacing post-shock or duringatrioventricular conduction block, or bradycardia pacing) or fordelivering a tachyarrhythmia induction sequence that includesentrainment pacing pulses prior to a T-shock or high frequency burstpulses (e.g., 50 Hz burst pulses). The methods disclosed herein forselecting a pacing output configuration may be used in conjunction withthe tachyarrhythmia induction methods generally disclosed in provisionalU.S. Patent Application 62/262,500 and corresponding U.S. PatentApplication Publication No. 2017/0157412, filed on the same dateherewith), both incorporated herein by reference in their entirety.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferior from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program cardiac rhythmdetection parameters and therapy control parameters used by ICD 14.Control parameters used to generate and deliver cardiac electricalstimulation pulses according to techniques disclosed herein may beprogrammed into ICD 14 using external device 40.

Data stored or acquired by ICD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand. For example, pacing capture threshold tests may be initiated bya user interacting with external device 40. A user may observe cardiacelectrical signals retrieved from ICD 14 on a display of external device40 for confirming cardiac capture by pacing pulses delivered by ICD 14during a capture threshold test. External device 40 may alternatively beembodied as a home monitor or hand held device.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted with ICDsystem 10 in a different implant configuration than the arrangementshown in FIGS. 1A-1B. FIG. 2A is a front view of patient 12 implantedwith ICD system 10. FIG. 2B is a side view of patient 12 implanted withICD system 10. FIG. 2C is a transverse view of patient 12 implanted withICD system 10. In this arrangement, lead 16 of system 10 is implanted atleast partially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22.In some instances, the anterior wall of anterior mediastinum 36 may alsobe formed by the transversus thoracis muscle and one or more costalcartilages. Anterior mediastinum 36 includes a quantity of looseconnective tissue (such as areolar tissue), adipose tissue, some lymphvessels, lymph glands, substernal musculature, small side branches ofthe internal thoracic artery or vein, and the thymus gland. In oneexample, the distal portion 25 of lead 16 extends along the posteriorside of sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36.

A lead implanted such that the distal portion 25 is substantially withinanterior mediastinum 36 may be referred to as a “substernal lead.” Inthe example illustrated in FIGS. 2A-2C, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. In some instances, lead 16 may extend laterally such that distalportion 25 of lead 16 is underneath/below the ribcage 32 in addition toor instead of sternum 22. In other examples, the distal portion 25 oflead 16 may be implanted in other extra-cardiovascular, intra-thoraciclocations, including the pleural cavity or around the perimeter of andadjacent to but typically not within the pericardium 38 of heart 26.Other implant locations and lead and electrode arrangements that may beused in conjunction with the cardiac pacing techniques described hereinare generally disclosed in the above-incorporated references. Althoughexample extra-cardiovascular locations are described above with respectto FIGS. 1A, 1B and 2A-2C, the pacing techniques of this disclosure maybe utilized in other implementations in which pacing amplitudes and/orwidths associated with conventional intra-cardiac pacing pulses areinsufficient to capture the patient's heart, including within thepericardial space.

FIG. 3 is a conceptual diagram illustrating a distal portion 25′ ofanother example of implantable electrical lead 16 having an alternativeelectrode arrangement. In this example, distal portion 25′ includes twopace/sense electrodes 28A and 28B and two defibrillation electrodes 24Aand 24B and respective conductors (not shown) to provide the electricalstimulation and sensing functionality as described above in conjunctionwith FIGS. 1A, 1B and FIGS. 2A-2C. In this example, however, electrode28B is proximal to proximal defibrillation electrode 24B, and electrode28A is distal to proximal defibrillation electrode 24B such thatelectrodes 28A and 28B are separated by defibrillation electrode 24B. Ina further example, in addition to electrodes 28A and 28B, lead 16 mayinclude a third pace/sense electrode located distal to defibrillationelectrode 24A.

The spacing and location of pace/sense electrodes 28A and 28B may beselected to provide pacing vectors that enable efficient pacing of heart26. The lengths and spacing of electrodes 24A, 24B, 28A and 28B maycorrespond to any of the examples provided in the above-incorporatedreferences. For example, the distal portion 25′ of lead 16 from thedistal end to the proximal side of the most proximal electrode (e.g.,electrode 28B in the example of FIG. 3) may be less than or equal to 15cm and may be less than or equal to 13 cm and or even less than or equalto 10 cm. The spacing and location of pace/sense electrodes 28A and 28Bmay be selected to provide pacing vectors that enable efficient pacingof heart 26. It is contemplated that one or more pace/sense electrodesmay be distal to distal defibrillation electrode 24A, one or morepace/sense electrodes may be between defibrillation electrodes 24A and24B, and/or one or more pace/sense electrodes may be proximal toproximal defibrillation electrode 24B. Having multiple pace/senseelectrodes at different locations along lead body 18 enables selectionfrom among a variety of inter-electrode spacings, which allows a pacingelectrode pair (or combination) to be selected having an inter-electrodespacing that results in the greatest pacing efficiency.

ICD 14 may deliver electrical stimulation and/or sense electricalsignals using any electrode vector that includes defibrillationelectrodes 24A and 24B (individually or collectively), and/or electrodes28A and/or 28B, and/or the housing 15 of ICD 14. For example, ICD 14 maydeliver pacing pulses using a low voltage therapy module via a pacingelectrode vector in which one of electrodes 28A or 28B is selected as acathode and the other of electrodes 28A and 28B is selected as theanode. Other examples of low-voltage therapy delivery electrode vectorsmay include one of electrodes 28A or 28B or both in combination selectedas a cathode (or anode) with one of defibrillation electrodes 24A, 24Bor housing 15 selected as an anode (or cathode). ICD 14 may deliverpacing pulses using a high voltage therapy module using a pacingelectrode vector that uses one or both of defibrillation electrodes 24Aand 24B as a cathode (or anode) and the housing 15 of ICD 14 as an anode(or cathode). ICD 14 is configured to determine which pacing vector andwhich one of a low voltage therapy module and a high voltage therapymodule are used to deliver cardiac pacing pulses, e.g., in accordancewith the techniques described herein.

FIG. 4 is a conceptual diagram illustrating a distal portion 25″ofanother example of extra-cardiovascular lead 16 having an electrodearrangement similar to that of FIG. 3 but with a non-linear or curvingdistal portion 25″ of lead body 18′. Lead body 18′ may be pre-formed tohave a normally curving, bending, serpentine, undulating, or zig-zaggingshape along distal portion 25″. In this example, defibrillationelectrodes 24A′ and 24B′ are carried along pre-formed curving portionsof the lead body 18′. Pace/sense electrode 28A′ is carried betweendefibrillation electrodes 24A′ and 24B′. Pace/sense electrode 28B′ iscarried proximal to the proximal defibrillation electrode 24B′.

In one example, lead body 18′ may be formed having a normally curvingdistal portion 25″ that includes two “C” shaped curves, which togethermay resemble the Greek letter epsilon, “ε” Defibrillation electrodes24A′ and 24B′ are each carried by the two respective C-shaped portionsof the lead body distal portion 25″ and extend or curve in the samedirection. In the example shown, pace/sense electrode 28A′ is proximalto the C-shaped portion carrying electrode 24A′, and pace/senseelectrode 28B′ is proximal to the C-shaped portion carrying electrode24B′. Pace/sense electrodes 24A′ and 24B′ are approximately aligned witha central axis 31 of the normally straight or linear, proximal portionof lead body 18′ such that mid-points of defibrillation electrodes 24A′and 24B′ are laterally offset from electrodes 28A′ and 28B′.Defibrillation electrodes 24A′ and 24B′ are located along respectiveC-shaped portions of the lead body distal portion 25″ that extendlaterally in the same direction away from central axis 31 and electrodes28A′ and 28B′. Other examples of extra-cardiovascular leads includingone or more defibrillation electrodes and one or more pacing and sensingelectrodes carried by curving serpentine, undulating or zig-zaggingdistal portion of the lead body that may be implemented with the pacingtechniques described herein are generally disclosed in pending U.S. Pat.Publication No. 2016/0158567 (Marshall, et al.), incorporated herein byreference in its entirety.

FIG. 5 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asa can electrode in FIG. 5) includes software, firmware and hardware thatcooperatively monitor one or more cardiac electrical signals, determinewhen a pacing therapy is necessary, and deliver prescribed pacingtherapies as needed. The software, firmware and hardware are alsoconfigured to determine when a CV/DF shock is necessary, and deliverprescribed CV/DF shock therapies. ICD 14 is coupled to anextra-cardiovascular lead, such as lead 16 carrying extra-cardiovascularelectrodes 24A, 24B, 28A, 28B and 30, for delivering pacing therapies,CV/DF shock therapies and sensing cardiac electrical signals.

ICD 14 includes a control module 80, memory 82, therapy delivery module84, electrical sensing module 86, and telemetry module 88. ICD 14 mayinclude an impedance measurement module 90 for delivering a drive signalacross a therapy delivery electrode vector and measuring a resultingvoltage for determining an electrical impedance of the electrode vector.

A power source 98 provides power to the circuitry of ICD 14, includingeach of the modules 80, 82, 84, 86, 88, 90 as needed. Power source 98may include one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 98 and each of the other modules 80, 82, 84, 86 and 88 areto be understood from the general block diagram of FIG. 5, but are notshown for the sake of clarity. For example, power source 98 is coupledto low voltage (LV) and high voltage (HV) charging circuits included intherapy delivery module 84 for charging LV and HV capacitors,respectively, or other energy storage devices included in therapydelivery module 84 for producing electrical stimulation pulses.

The functional blocks shown in FIG. 5 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. As used herein,the term “module” refers to an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsthat provide the described functionality. The particular form ofsoftware, hardware and/or firmware employed to implement thefunctionality disclosed herein will be determined primarily by theparticular system architecture employed in the device and by theparticular detection and therapy delivery methodologies employed by theICD 14. Providing software, hardware, and/or firmware to accomplish thedescribed functionality in the context of any modern ICD system, giventhe disclosure herein, is within the abilities of one of skill in theart.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control module 80 orother ICD modules to perform various functions attributed to ICD 14 orthose ICD modules. The non-transitory computer-readable media storingthe instructions may include any of the media listed above.

The functions attributed to the modules herein may be embodied as one ormore processors, hardware, firmware, software, or any combinationthereof. Depiction of different features as modules is intended tohighlight different functional aspects and does not necessarily implythat such modules must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modulesmay be performed by separate hardware, firmware or software components,or integrated within common hardware, firmware or software components.For example, cardiac pacing operations may be performed by therapydelivery module 84 under the control of control module 80 and mayinclude operations implemented in a processor executing instructionsstored in memory 82.

Control module 80 communicates with therapy delivery module 84 andelectrical sensing module 86 for sensing cardiac electrical activity,detecting cardiac rhythms, and controlling delivery of cardiacelectrical stimulation therapies in response to sensed cardiac signals.Therapy delivery module 84 and electrical sensing module 86 areelectrically coupled to electrodes 24A, 24B, 28A, 28B, and 30 carried bylead 16 (shown in FIG. 1A and 1B) and the housing 15, which may functionas a common or ground electrode or as an active can electrode fordelivering CV/DF shock pulses.

Electrical sensing module 86 may be selectively coupled to electrodes28A, 28B, 30 and housing 15 in order to monitor electrical activity ofthe patient's heart. Electrical sensing module 86 may additionally beselectively coupled to electrodes 24A and/or 24B. Sensing module 86 isenabled to selectively monitor one or more sensing vectors selected fromthe available electrodes 24A, 24B, 28A, 28B, 30 and housing 15. Forexample, sensing module 86 may include switching circuitry for selectingwhich of electrodes 24A, 24B, 28A, 28B, 30 and housing 15 are coupled tosense amplifiers or other cardiac event detection circuitry included insensing module 86. Switching circuitry may include a switch array,switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple sense amplifiers to selected electrodes.The cardiac event detection circuitry within electrical sensing module86 may include one or more sense amplifiers, filters, rectifiers,threshold detectors, comparators, analog-to-digital converters (ADCs),or other analog or digital components.

In some examples, electrical sensing module 86 includes multiple sensingchannels for acquiring cardiac electrical signals from multiple sensingvectors selected from electrodes 24A, 24B, 28A, 28B, 30 and housing 15.Each sensing channel may be configured to amplify, filter and rectifythe cardiac electrical signal received from selected electrodes coupledto the respective sensing channel to improve the signal quality forsensing cardiac events, e.g., P-waves and/or R-waves. Each sensingchannel includes cardiac event detection circuitry for sensing cardiacevents from the received cardiac electrical signal developed across theselected sensing electrode vector(s). For example, each sensing channelin sensing module 86 may include an input or pre-filter and amplifierfor receiving a cardiac electrical signal from a respective sensingvector, an analog-to-digital converter, a post-amplifier and filter, arectifier to produce a digitized, rectified and amplified cardiacelectrical signal that is passed to a cardiac event detector included insensing module 86 and/or to control module 80. The cardiac eventdetector may include a sense amplifier, comparator or other circuitryfor comparing the rectified cardiac electrical signal to a cardiac eventsensing threshold, such as an R-wave sensing threshold, which may be anauto-adjusting threshold. Sensing module 84 may produce a sensed cardiacevent signal in response to a sensing threshold crossing. The sensedcardiac events, e.g., R-waves, are used for detecting cardiac rhythmsand determining a need for therapy by control module 80. In someexamples, cardiac electrical signals such as sensed R-waves are used todetect capture of a pacing pulse delivered by ICD 14.

Therapy delivery module 84 includes a low voltage (LV) therapy module 85for delivering low voltage pacing pulses using an extra-cardiovascularpacing electrode vector selected from electrodes 24A, 24B, 28A, 28B, 30and 15. LV therapy module 85 may be configured to deliver low voltagepacing pulses, e.g., 8 V or less or 10 V or less or 15 V or less or 18 Vor less. One or more capacitors included in the LV therapy module 85 arecharged to a voltage according to a programmed pacing pulse amplitude bya LV charging circuit, which may include a state machine. At anappropriate time, the LV therapy module 85 couples the capacitor(s) to apacing electrode vector to deliver a pacing pulse to the heart 26.

LV therapy module 85 is capable of operating in one or more low-voltagepacing output configurations. In one example, LV therapy module 85 maybe enabled to deliver low-voltage, single pulse pacing pulses in a firstlow-voltage pacing output configuration. When the capture threshold ofheart 26 is higher than a maximum single-pulse pacing pulse outputproducible by LV therapy module 85, LV therapy module 85 may be enabledto deliver composite pacing pulses comprising two or more fusedindividual pulses in a second low-voltage pacing output configuration.The fused pacing pulses are delivered by sequentially discharging atleast two holding capacitors or at least two holding capacitorcombinations included in LV therapy module 85 to deliver at least twoindividual pulses that are fused in time to produce a composite pacingpulse having a greater pulse energy than a maximum pulse energy of asingle pulse pacing pulse producible by LV therapy module 84. Forexample, the total pulse width of a fused pacing pulse is longer thanthe maximum single-pulse pacing pulse width that is producible by LVtherapy module 84. When a single-pulse pacing pulse of a given voltageamplitude does not capture the heart, even at the maximum availablepulse amplitude and width of the single pulse, a composite pacing pulsehaving the same voltage amplitude has a total pulse energy deliveredover the composite pacing pulse width that may be greater than thecapture threshold of the heart. The pulse voltage amplitude may be themaximum voltage amplitude tolerable by the patient or the maximum pulseamplitude (which may be 8 V or 10 V or 15 V or 18V in some examples)available from LV therapy module 84.

In other examples, LV therapy module 85 may only operate in onelow-voltage pacing output configuration, e.g., only the single pulselow-voltage pacing output configuration, only the low-voltage fusedpacing output configuration, or some other low-voltage pacing outputconfiguration. In another example, LV therapy module 85 may be capableof operating in more than two different selectable low-voltage pacingoutput configurations.

High voltage (HV) therapy module 83 includes one or more high voltagecapacitors. When a shockable rhythm is detected, the HV capacitor(s)is(are) charged to a voltage level by a HV charging circuit according tothe programmed shock energy. The HV charging circuit may include atransformer and be a processor-controlled charging circuit that iscontrolled by control module 80. Control module 80 applies a signal totrigger discharge of the HV capacitor(s) upon detecting a feedbacksignal from therapy delivery module 84 that the HV capacitors havereached the voltage required to deliver the programmed shock energy. Inthis way, control module 80 controls operation of the high voltagetherapy module 83 to deliver CV/DF shocks using defibrillationelectrodes 24A, 24B and/or housing 15.

HV therapy module 83 may be used to deliver cardiac pacing pulses whenpacing pulses delivered by LV therapy module 85 in a low-voltage pacingoutput configuration do not adequately capture heart 26. In this case,the HV capacitor(s) is(are) charged to a much lower voltage than thatused for delivering shock therapies but may be higher than the maximumavailable pulse amplitude produced by the LV therapy module 85. Forexample, the HV capacitor may be charged to 40 V or less, 30 V or less,or 20 V or less for producing extra-cardiovascular pacing pulses. Inmost instances, the HV circuitry is generally designed for delivery ofthe high-voltage CV/DF shocks which are typically associated withvoltages that are much higher than the 40 V, 30V, or 20V. For example,the voltages associated with CV/DF shocks may be at least ten timesgreater than those voltages. The HV circuitry of therapy delivery module84 may only be capable of producing reduced level voltages to a certainminimum level. The minimum level may be 10V in one example. In otherexamples the minimum voltage level may be 15V or even 20V depending onthe design.

Compared to low-voltage pacing output configurations, a longer pulsewidth may be utilized in the high-voltage pacing output configurationwhile still maintaining a pulse voltage amplitude that is greater thanthe pacing capture threshold when discharging the HV capacitor(s). Thelonger pulse width is attainable due to a higher capacitance (andconsequently higher RC time constant) of the HV capacitor(s). As such,the LV therapy module 85 may be capable of producing a maximum pulsevoltage amplitude of up to and including 10 V. The maximum single-pulsepacing pulse width may be 2 ms. The maximum composite pacing pulse widthmay be up to 8 ms or higher.

The HV therapy module 83 may be capable of producing a pulse voltageamplitude of at least the minimum voltage level attainable by the HVcircuitry (e.g., 10 V or more, 15 V or more, 20 V or more). The minimumvoltage level may, in some examples, be greater than the maximum voltagelevel of LV therapy module 85. In other examples there may be an overlapof the maximum voltage level of LV therapy module 85 (e.g., 8V in oneexample) and the minimum voltage level attainable by the HV therapymodule 83 (e.g., 15-20 V in one example). The HV therapy module 83 mayalso produce mono- or multi-phasic pulses having a relatively longerpacing pulse width, e.g., 10 ms or more, because of the highercapacitance of high voltage capacitors included in HV circuitry. Atypical HV pacing pulse width may be 10 ms; however an example range ofavailable pulse widths may be 2 ms to 20 ms. An example of a maximumvoltage amplitude that may be used for delivering high voltage pacingpulses may be 40 V. When a relatively higher pacing pulse voltageamplitude is tolerable by the patient, e.g., more than 10 V, arelatively shorter pacing pulse width, e.g., 2 to 5 ms, may be usedduring the high-voltage pacing output configuration. However, a longerpacing pulse width may be used as needed, e.g., a 10 V, 20 ms pacingpulse.

For the sake of comparison, the HV capacitor(s) of the HV therapy module83 may be charged to an effective voltage greater than 100 V fordelivering a cardioversion/defibrillation shock. For example, two orthree HV capacitors may be provided in series having an effectivecapacitance of 148 microfarads in HV therapy module 83. These seriescapacitors may be charged to develop 750 to 800 V for the seriescombination in order to deliver shocks having a pulse energy of 5 Joulesor more, and more typically 20 Joules or more. The pacing pulsesdelivered by the HV therapy module 83 will have a pulse energy in themilliJoule range or at least tenths of milliJoules. For instance, apacing pulse generated by HV therapy module 83 having a 10 V amplitudeand 20 ms pulse width may be in the range of 2 to 5 milliJoules when thepacing electrode vector impedance is in the range of 400 to 1000 ohms.

Composite pacing pulses, delivered by the LV therapy module 85, havingan 8 V amplitude and 8 ms pulse width may be in the range of 0.5 to 1.3milliJoules for a similar range of pacing loads as given in thepreceding example. Extra-cardiovascular, single-pulse pacing pulsesdelivered by LV therapy module 83 that are 8V in amplitude and 2 ms inpulse width may be in the range of 0.2 to 0.3 milliJoules for pacingloads of 400 to 1000 ohms. In contrast, pacing pulses delivered usingendocardial electrodes or epicardial electrodes may be on the order ofmicroJoules, e.g., 2 microJoules to 5 microJoules for a typicalendocardial pacing pulse that is 2V in amplitude, 0.5 ms in pulse widthand applied across a pacing electrode vector impedance of 400 to 1000ohms.

As will be described below, control module 80 may enable a high-voltagepacing output configuration using HV therapy module 83 by applying atleast a minimum electrical current required to enable switchingcircuitry included in HV therapy module 83 for coupling the HVcapacitor(s) to a pacing electrode vector. Circuitry included in HVtherapy module 83 and LV therapy module 85 is also described inconjunction with FIGS. 9 and 11 respectively.

In some instances, control module 80 may control impedance measurementmodule 90 to determine the impedance of a pacing electrode vector.Impedance measurement module 90 may be electrically coupled to theavailable electrodes 24A, 24B, 28A, 28B, 30 and housing 15 forperforming impedance measurements of one or more candidate pacingelectrode vectors. Control module 80 may control impedance measurementmodule 90 to perform impedance measurements by passing a signal toimpedance measurement module 90 to initiate an impedance measurement ofa pacing electrode vector. Impedance measurement module 90 is configuredto apply a drive or excitation current across a pacing electrode vectorand determine the resulting voltage. The voltage signal may be useddirectly as the impedance measurement or impedance may be determinedfrom the applied current and the measured voltage. The impedancemeasurement may be passed to control module 80.

As described in conjunction with FIG. 10 below, control module 80 mayuse the impedance measurement to set a variable shunt resistanceincluded in HV therapy module 83 when a high-voltage pacing outputconfiguration is selected for delivering extra-cardiovascular pacingpulses to heart 26. The variable shunt resistance may be parallel to thepacing load and set to be equal to or less than the pacing loadimpedance to maintain electrical current through HV therapy moduleswitching circuitry throughout the duration of a pacing pulse deliveredby the HV therapy module 83 thereby promoting an appropriate voltagesignal across the pacing load for capturing the patient's heart.

Control parameters utilized by control module 80 for detecting cardiacrhythms and delivering electrical stimulation therapies andtachyarrhythmia induction pulses may be programmed into memory 82 viatelemetry module 88. Telemetry module 88 includes a transceiver andantenna for communicating with external device 40 (shown in FIG. 1A)using RF communication as described above. Under the control of controlmodule 80, telemetry module 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrymodule 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

FIG. 6 is a depiction of one example of a composite pacing pulse 50 thatmay be generated and delivered by LV therapy module 85 of ICD 14 to paceheart 26 using extra-cardiovascular electrodes when a low-voltage, fusedpacing output configuration is selected by control module 80. Pacingpulse 50 is a composite pacing pulse comprising four pulses 52 a, 52 b,52 c, and 52 d that are each individually delivered by discharging a LVholding capacitor (or a combination of parallel LV holding capacitors)across a selected pacing electrode vector via a respective outputcapacitor included in a capacitor array of LV therapy module 85.Discharging a single LV holding capacitor (or combination of parallel LVholding capacitors simultaneously) results in delivery of a single,low-voltage pacing pulse by LV therapy module 85. In one low-voltagepacing output configuration, LV therapy module 85 is enabled todischarge multiple LV holding capacitors in a sequential manner across aselected pacing electrode vector such that multiple single low-voltagepacing pulses are delivered sequentially to produce a composite pacingpulse 50 that elicits a single evoked response by the myocardium.

The first pulse 52 a defines a leading edge 58 a of the composite pulse50. Each of the pulses 52 a-52 d has a peak voltage amplitude 66according to a programmed pulse amplitude. A decaying portion 56 a, 56b, 56 c, and 56 d of each respective pulse decays according to an RCtime constant of the discharge circuit of LV therapy module 85. Eachindividual pulse 52 a-52 d may be truncated at an individual pulse width62. The leading edge 58 b, 58 c and 58 d of the respective pulses 52 b,52 c and 52 d coincides in time with the terminating edge 60 a, 60 b and60 c, respectively of the immediately preceding pulse, 52 a, 52 b, and52 c, respectively. The terminating edge 60 d of the final pulse 52 ddefines the trailing edge of the composite pulse 50.

The composite pulse 50 has a time-varying pulse amplitude that reachesthe maximum pulse amplitude 66 at the leading edge 58 a-58 d of eachpulse with periods of decay between the leading edges 58 a-58 d to theminimum pulse amplitude 68 just prior to the next leading edge. Theindividual pulse width 62 may be set to maintain the minimum pulseamplitude 68 of each individual pulse 52 a-52 d just prior to the nextindividual pulse above a minimum amplitude threshold to ensure that thetotal pulse energy delivered in the composite pulse 50 successfullycaptures and paces the heart 26. The individual pulse width 62 may befixed, e.g., up to 2 ms in some examples so that the total pulse widthis up to 8 ms when four fused, consecutive pulses 52 a-52 b aredelivered as shown in the example of FIG. 6. The individual pulse width62 may be a maximum pulse width available from LV therapy module 85 whena single holding capacitor is discharged across the pacing electrodevector. This maximum single pulse width may be limited by the decay rateof the pacing pulse as determined by the RC time constant of thedischarge circuit. For a given pacing pulse width, a pacing capturethreshold amplitude can be identified as the lowest pacing pulseamplitude that elicits an evoked response (depolarization) of themyocardial tissue. The decay rate of a single pacing pulse determined bythe RC time constant may cause the amplitude of the pacing pulse to fallbelow a myocardial pacing capture threshold amplitude before the singlepacing pulse width that is required to achieve myocardial capture forthe given pacing pulse amplitude can be reached. If the pacing pulseamplitude is set to a maximum available value, e.g., 8 Volts or 10Volts, the maximum pacing pulse width of a single pacing pulse asdetermined by the RC time constant may be insufficient to deliver thepulse energy required to capture the heart using a selectedextra-cardiovascular pacing electrode vector. As such, a compositepacing pulse, having a total pulse width that is greater than themaximum available pulse width of a single pacing pulse, may be requiredto achieve a total pulse energy sufficient to capture the heart.

The individually delivered pulses are fused in time such that theindividual pulse energy is cumulative in producing a total pulse energythat is greater than the pacing capture threshold of the patient'sheart, even though each individual pulse 52 a-52 d may have a pulseenergy that is less than the pacing capture threshold. The number ofindividual pulses delivered (which may be more or less than four in someexamples) in fused sequence may be selected based on the total pacingpulse width 64 required to capture the heart for a given pulse amplitude66, and the maximum individual pulse width 62 that can be reachedwithout allowing the minimum pulse amplitude 68 to fall below a minimumamplitude threshold for a time period between the fused pulses thatprevents the pulses from having a cumulative dose effect for capturingthe myocardium. For example, the minimum pulse amplitude 68 may beprevented from reaching 0 V between individual pulses and may bemaintained above an amplitude threshold, which may be defined as apercentage of the programmed pulse amplitude 66, e.g., 25%, 50% or otherselected percentage of programmed pulse amplitude 66.

The terminating edges 60 a, 60 b, and 60 c of respective pulses 52 a-52c occur nearly simultaneously with leading edges 58 b, 58 c, and 58 d ofthe respective subsequent pulses 52 b, 52 c, and 52 d within the limitsof the electronic circuitry. It is recognized that limitations withinthe electronic circuitry may result in a non-zero time gap betweenindividual pulses 52 a-52 d in some examples. The delivered energy ofeach individual pulse 52 a-52 d, however, is fused close enough in timeto a preceding and/or subsequent individual pulse such that theindividual pulse energies accumulate to achieve a dose responsenecessary to achieve capture of the patient's heart. Each individualpulse 52 a-52 d may have a pulse energy below the capture threshold ofthe heart. By delivering the individual pulses 52 a-52 d within a timewindow defined by the total pulse width 64, the total composite pacingpulse energy that is delivered is greater than the pacing capturethreshold of the heart. As such, the composite pulse captures the hearteven when each individual pulse 52 a-52 d delivered alone or spacedfurther apart in time may be insufficient to capture and pace the heart.

Each individual pulse 52 a-52 d may be delivered across the pacingelectrode vector having the same polarity (positive-going in the exampleshown) by sequentially coupling different capacitance elements (a singlecapacitor or a combination of two or more capacitors) across theselected pacing electrode vector. Each of the different capacitanceelements are charged to the peak voltage amplitude 66 prior to beingcoupled across the pacing electrode vector. In some examples, the samecapacitor or combination of capacitors may not be used to deliver twoconsecutive individual pulses, e.g., pulses 52 a and 52 b, sincecharging of the capacitor (or combination of capacitors) to the peakvoltage amplitude 66 occurs prior to initiating each respective one ofthe individual pulses 52 a-52 d. The same capacitor or same combinationof capacitors may be used to deliver two non-consecutive individualpulses, e.g., 52 a and 52 d, by recharging the same capacitor orcombination of capacitors to the peak voltage amplitude 66 during theintervening one or more individual pulses 52 b and 52 c.

Each individual pulses 52 a-52 d is shown to have the same peak voltageamplitude 66 in FIG. 6. The peak voltage amplitude may be the maximumvoltage amplitude available from the LV therapy module 85 or a maximumvoltage amplitude tolerable by the patient. The total pulse energy ofthe composite pacing pulse 50 may be controlled by setting theindividual pulse number and individual pulse width of pulses 52 a-52 d.It is contemplated, however, that one capacitor (or combination ofcapacitors) that is discharged to deliver one of the individual pulses52 a-52 d may be charged to a different voltage than another capacitor(or combination of capacitors) used to deliver a different one of theindividual pulses 52 a-52 d. As a result, the individual pulses 52 a-52d may have different peak voltage amplitudes (and/or polarity) in someinstances. Individual pulses 52 a-52 d, however, are generated byswitching out a first discharging capacitor (or combination ofcapacitors) and switching in a next capacitor (or combination ofcapacitors) that is(are) charged to the desired peak voltage amplitudeof the next individual pulse. A first individual pulse is therebyterminated by stopping discharging of the first capacitor(s), and thenext individual pulse is started by starting discharging of the nextcapacitor(s).

In some examples, the pacing pulse amplitude may be monitored in realtime during the delivery of composite pacing pulse 50, and, when thedecaying amplitude drops to an amplitude threshold value, the nextindividual pulse is started. For example, the amplitude of decayingportion 56 a may be sampled, and when the minimum amplitude 68 isreached the next pulse 52 b is started. The first pulse 52 a istruncated when the next pulse 52 b is started so that terminating edge60 a of pulse 52 a and leading edge 58 b of the second pulse 52 b occursimultaneously. Pacing pulse 50 is followed by a recharge pulse 69comprising a low amplitude pulse in opposite polarity for each of theindividual pulses 52 a-52 d. The recharge pulse 69 may allow an outputcapacitor of the LV therapy module 85 to passively discharge if it hascharged during the delivery of pacing pulse 50 to promote chargeneutrality and may reduce polarization artifact of the pacingelectrodes.

In other examples, the individual pulses 52 a through 52 d of compositepacing pulse 50 may be overlapping in that the leading edge 58 b, 58 cor 58 d of pulses 52 b, 52 c and 52 d, respectively, may slightlyprecede the respective terminating edge 60 a, 60 b or 60 c of therespective preceding pulses 52 a, 52 b and 52 c. While individual pulses52 a-52 d are shown each having an equal individual pulse width 62, itis contemplated that individual pulses 52 a-52 d may have differingindividual pulse widths, which may be based on the effective capacitanceof a holding capacitor or holding capacitor combination that is beingdischarged to produce the individual pulses 52 a-52 d. For example, acomposite pacing pulse of 8.0 ms may be produced by delivering twoindividual pulses each having a pulse width of 2.0 ms by discharging twodifferent holding capacitors in a sequential manner followed by a thirdindividual pulse having a pulse width of 4.0 ms produced by dischargingtwo parallel holding capacitors simultaneously. A composite pulse ofthis type and other techniques for producing a composite pacing pulseincluding multiple individual pulses delivered in fused succession aregenerally disclosed in provisional U.S. Pat. Application No. 62/262,412and the corresponding U.S. Pat. Application Publication No. 2017/0157399filed on the same day herewith), the content of which is incorporatedherein by reference in its entirety.

The composite pacing pulse 50 is produced to provide a pacing pulsehaving a total pulse energy within the total composite pacing pulsewidth 64 that is adequate to capture the heart by producing an evokeddepolarization response of the heart. As described below, a low-voltage,fused pacing output configuration may be selected by control module 80when a low-voltage single pulse pacing output configuration does notsatisfy pacing capture threshold criteria. Alternatively, the LVcomposite pacing pulse output configuration may be the only LV pacingoutput configuration.

FIG. 7 is a depiction of one example of a high voltage pacing pulse 70that may be generated and delivered by HV therapy module 83 of ICD 14 topace heart 26 using extra-cardiovascular electrodes when a high-voltagepacing output configuration is selected by control module 80. When lowvoltage pacing pulses delivered by LV therapy module 85 are determinedto inadequately capture heart 26, by either a low-voltage, single-pulsepacing output configuration or a low-voltage, fused pacing outputconfiguration, control module 80 is configured to automatically enableHV therapy module 83 to deliver cardiac pacing pulses. HV therapy module83 may produce a high voltage pacing pulse 70 having a programmed pacingpulse amplitude 72 that is greater than the maximum voltage amplitudethat LV therapy module 85 can produce but is much less than the voltageamplitude of CV/DF shock pulses required to cardiovert or defibrillatethe heart 26. For example, high voltage pacing pulse amplitude 72 may begreater than or equal to 10 V and up to 40 V, inclusive, or may be from10 V to 30 V in other examples. In another example, high voltage pacingpulse amplitude is greater than 8 V, up to and including 40 V. The highvoltage pacing pulse 70 may have a pulse energy that is less than adefibrillation threshold of the heart 26. In the example shown, pacingpulse 70 has a pulse voltage amplitude of 10 V and a pulse width 74 of20 ms. In another example, pacing pulse 70 has a pulse voltage amplitudeequal to or between 10 and 20 V and a pulse width of 10 ms.

The pulse width 74 may depend on the pacing pulse amplitude 72 such thatthe total pacing pulse energy delivered by pulse 70 having amplitude 72and width 74 successfully captures and paces heart 26, but may be lessthan a defibrillation threshold. In some examples, the pulse width 74may be from 1 ms up to and including 10 ms, but may be shorter than orlonger than this example range. The pulse width 74 may be set accordingto a pulse width threshold determined for the programmed pacing pulseamplitude 72. For example, if the pulse amplitude is set to 20 V, theminimum pulse width that successfully captures the heart may bedetermined during a pacing capture threshold test. Pacing pulse width 74may be set at a safety pacing margin longer than the capture thresholdpulse width. Alternatively, pulse width 74 may be set first and pulseamplitude 72 may be set to an amplitude that is a safety margin abovethe capture threshold pulse amplitude found during a pacing capturethreshold test using the selected pulse width 74 when HV therapy module83 is enabled to deliver pacing pulses in a high-voltage pacing outputconfiguration.

As shown, pacing pulse 70 is a biphasic pacing pulse having a first,positive-going portion 70 a and a second, negative going portion 70 b. Abiphasic pacing pulse 70 may be produced by HV therapy module 83 throughthe control of switching circuitry included in HV therapy module 83.Switching circuitry of HV therapy module 83 may controlled to reversethe polarity of the delivered pulse during capacitor discharging toproduce the biphasic pulse. The polarity may be reversed at a givenvoltage threshold in some examples. The HV capacitor charged to thepulse voltage amplitude 72 continues to be discharged for the remainingportion 70 b of pacing pulse width 74. As can be observed in FIG. 7, theending voltage amplitude of the positive-going portion 70 a is thestarting voltage amplitude of the negative-going portion 70 b becausethe same capacitor(s) continue to be discharged after switchingcircuitry reverses the polarity of the pacing pulse 70. In contrast, theindividual pulses 52 a-52 d of the composite pacing pulse 50 of FIG. 6are each produced by switching to a different capacitor (or combinationof capacitors) that has been charged to the pulse voltage amplitude 66to begin the next individual pulse. Each individual pulse 52 a-52 d isproduced by discharging a different capacitor (or combination ofcapacitors) across the pacing electrode vector resulting in individualpulses each having a leading pulse amplitude equal to the pulseamplitude 66. In other examples, high voltage pacing pulse 70 may bedelivered as a monophasic, other multi-phasic, or other shaped pulsethrough the control of the switching circuitry.

FIG. 8 is a flow chart 100 of one method for selecting a pacing outputconfiguration for use in delivering extra-cardiovascular cardiac pacingpulses by ICD 14. In some patients, capture of heart 26 may beachievable using single, non-fused low voltage pacing pulses. In thiscase, control module 80 may configure the pacing output configuration tobe a low-voltage, single pulse pacing output configuration using LVtherapy module 85. In other patients, the pacing capture threshold maybe greater than the maximum output of a single pulse produced by LVtherapy module 85. In other words, the pacing capture threshold isgreater than the maximum available pulse amplitude and/or greater thanthe maximum available single pulse width produced by LV therapy module85, e.g., when a single holding capacitor is discharged across a pacingelectrode vector. In that case, control module 80 enables ahigher-energy pacing pulse output configuration than the low-voltage,single pulse pacing output configuration.

The higher energy pacing pulse output configurations available mayinclude a low-voltage, fused pacing pulse output configuration in whichcomposite pacing pulses, such as composite pacing pulse 50 shown in FIG.6, are delivered by LV therapy module 85. Another higher energy pacingpulse output configuration may be a high-voltage pacing outputconfiguration in which high voltage pacing pulses, e.g., pulse 70 shownin FIG. 7, are delivered by HV therapy module 83.

As used herein, a “high voltage pacing pulse” is a pacing pulse producedby the HV therapy module 83 by discharging a HV capacitor that ischarged to a voltage that is greater than a maximum voltage that a LVcapacitor including in LV therapy module 85 can be charged to. Asdescribed below, the HV therapy module 83 includes a HV charging circuithaving a transformer used to increase the voltage available for chargingthe HV capacitor under the control of a processor. In comparison, the LVtherapy module 85 includes a LV charging circuit that may be controlledby a state machine and uses a multiple of the battery voltage of powersource 98, e.g., four times the battery voltage.

In order to determine the most appropriate pacing output configuration,pacing capture threshold tests using one or more of the available pacingoutput configurations may be performed. The pacing capture thresholdtest may be performed upon ICD implantation, on a scheduled periodicbasis, upon detecting or predicting a heart rhythm requiring therapy,detecting loss of capture, or detecting a lead issue or other conditionthat may be indicative of a loss of capture. Control module 80determines an appropriate pacing output configuration that is capable ofreliably capturing and pacing the heart while minimizing the energyrequired to produce pacing pulses and minimizing patient discomfortcaused by the extra-cardiovascular pacing pulses. Control module 80selects the pacing output configuration from among at least onelow-voltage pacing output configuration that uses LV therapy module 85and one high-voltage pacing output configuration that uses HV therapymodule 83.

The low-voltage pacing output configuration(s) and the high voltagepacing output configuration may deliver pacing pulses using a pacingelectrode vector selected from any of the extra-cardiovascularelectrodes carried by an extra-cardiovascular lead coupled to ICD 14,e.g., lead 16 shown and described in conjunction with FIGS. 1A-4. In oneexample, the low-voltage pacing output configuration delivers pacingpulses via one or more of electrodes 28A or 28B, or the housing 15,e.g., between one of electrodes 28A and 28B as a cathode (or anode) andthe housing electrode as an anode (or cathode) or between electrode 28Aas a cathode (or anode) and electrode 28B as an anode (or cathode). Thehigh-voltage pacing output configuration may deliver pacing pulses viathe defibrillation electrodes 24A and 24B, one serving as the anode (orcathode) and the other as the cathode (or anode), or between one of thedefibrillation electrodes 24A or 24B paired with the housing 15. Thelow-voltage and high-voltage pacing output configurations are notlimited for use with a particular pacing electrode vector. The pacingelectrode vectors used for the low-voltage pacing output configurationand the high-voltage pacing output configuration may be selected basedon individual patient need and the particular lead and electrodeconfiguration being used and its placement in the patient's body.

Beginning at block 102, control module 80 may start a low-voltage,single pulse pacing capture threshold test. LV therapy module 85 may becontrolled to deliver low-voltage, single-pulse pacing pulses accordingto a capture threshold test algorithm in order to determine ifsuccessful capture can be achieved using a pulse amplitude and pulsewidth that are within acceptable capture threshold limits for using thelow-voltage, single pulse pacing output configuration. The LVsingle-pulse pacing capture threshold test performed at block 102 may beperformed automatically by ICD 14 when control module 80 is configuredto control LV therapy module 85 to deliver pacing pulses according to acapture threshold search algorithm including automatic capture detectionby control module 80. In other examples, the LV, single-pulse pacingthreshold test, and other capture threshold tests described herein, maybe performed in a semi-automatic way in response to user-enteredprogramming commands received from external device 40.

In some examples, the pacing capture threshold test begins at block 102with a relatively low pacing pulse amplitude, e.g., 1 V, and a maximumavailable pulse width, e.g., 1.5 ms or 2.0 ms. Capture may be detectedby control module 80 automatically, for example based on detecting anevoked R-wave from a cardiac electrical signal received by sensingmodule 86 within an expected time interval after delivering a pacingpulse. Alternatively, capture may be verified manually by a userobserving an ECG signal provided by sensing module 86 and transmitted toexternal device 40 by telemetry module 88. The user may enter a commandinto external device 40 transmitted back to ICD 14 indicating thatcapture is confirmed.

If capture is not detected or verified at the starting pulse amplitudeand maximum pulse width, the pulse amplitude may be increased in 1 Vincrements or other voltage steps until capture is achieved or until amaximum test pulse amplitude is reached. In some examples, the maximumtest pulse amplitude is the maximum pulse amplitude available from theLV therapy module 85. In other examples, the maximum test pulseamplitude is the maximum amplitude available from the LV therapy module85 minus a predetermined safety margin. If capture cannot be achieved ata pulse amplitude that is at least a safety margin below the maximumavailable pulse amplitude, the capture threshold may be consideredunacceptable for extra-cardiovascular pacing using the low-voltage,single pulse pacing output configuration.

In one example, the programmed safety margin is set equal to the pacingcapture threshold for extra-cardiovascular pacing. In other words, thefinal pacing pulse amplitude is set to double the capture thresholdvoltage amplitude. For instance, if the capture threshold is 3 V, thesafety margin is also 3 V so that the pacing pulse amplitude is set to 6V. If the pacing capture threshold amplitude is determined to be 5 V,the required safety margin is 5 V resulting in 10 V for the pacing pulseamplitude, which may exceed the maximum available pulse amplitude of LVtherapy module 85. In other examples, the safety margin may be a fixedincrement greater than the pulse amplitude capture threshold, forexample 2 V greater than the pacing amplitude capture threshold. If thepacing capture threshold plus the safety margin is equal to or exceedsthe maximum available pulse amplitude available for the LV therapymodule 85, the pacing capture threshold may be determined to beunacceptable for low-voltage, single pulse pacing at block 104. Thecontrol module 80 advances to block 110 to perform a pacing thresholdtest for low-voltage fused pacing pulses.

In other examples, control module 80 may begin the LV pacing thresholdtest performed at block 102 by controlling LV therapy module 85 todeliver a test pacing pulse at a maximum output available for alow-voltage, single pulse. For example, the maximum output may be apulse that is 8V in amplitude and 1.5 ms or 2.0 ms in pulse width.Control module 80 determines if this maximum-output pacing pulsecaptures the heart 26 based on detection of an R-wave following a pacingpulse from the cardiac electrical signal received by sensing module 86or receipt of manual confirmation by a user. If a single-pulse pacingpulse delivered at the maximum output of LV therapy module 85, i.e.,maximum available pulse amplitude and maximum available pulse width,does not capture, the pacing capture threshold is greater than theavailable pulse energy of a low-voltage, single-pulse, pacing outputconfiguration. The capture threshold is not acceptable for implementingthis output configuration as determined at block 104. Control module 80advances to block 110 to perform a pacing threshold test using alow-voltage, fused pacing output configuration.

If the maximum pulse output does capture the heart during the LV pacingthreshold test performed at block 102, the pacing pulse output may bedecreased by decreasing the pulse amplitude and/or the pulse width in anincremental manner until loss of capture is detected by control module80 based on cardiac electrical signals received by sensing module 86 (ormanual confirmation). In other examples, the pacing threshold testperformed at block 104 may include increasing pulse width from astarting pulse width and/or decreasing pulse width from a starting pulsewidth. For example, pulse amplitude may be increased from a startingpulse amplitude that is at least a safety margin below the maximumavailable pulse amplitude, and pacing pulses may be delivered at a giventest voltage amplitude for one or more pulse widths. If the pacing pulseamplitude reaches a maximum test amplitude and capture has not beendetected, the pulse width may be increased from a starting pulse widthup to a maximum test pulse width. The pacing capture threshold may bedetermined as the minimum pulse amplitude for a given pulse width or theminimum pulse width for a given pulse amplitude that results in capture.

Control module 80 compares the pacing capture threshold to acceptablethreshold criteria at block 104. Acceptable threshold criteria appliedto the threshold test results may include a maximum acceptable pulseamplitude and/or a maximum acceptable pulse width. For example, if thecapture threshold is not at least an amplitude safety margin below themaximum available pulse amplitude and/or a time interval safety marginbelow a maximum available pulse width, the capture threshold isunacceptable for the low-voltage, single-pulse pacing outputconfiguration. In one example, acceptable capture threshold criteriaincludes a pulse amplitude capture threshold that is less than half themaximum pacing pulse amplitude available from the LV therapy module 85when the pulse width is the maximum pulse width available from the LVtherapy module 85. To illustrate, if the maximum output capability of LVtherapy module 85 is an 8 V, 2.0 ms pacing pulse, the acceptable capturethreshold criteria requires successful capture at a pulse amplitude thatis 4 V or less when the pulse width is 2.0 ms.

If the pacing capture threshold determined at block 102 satisfies theacceptable capture threshold criteria applied at block 104, controlmodule 80 enables the low-voltage, single-pulse pacing from the LVtherapy module at block 106. LV pacing parameters are set at block 108for controlling extra-cardiovascular pacing delivered by LV therapymodule 85. The LV pacing parameters set at block 104 may be based on thecapture threshold results. For example, the pacing pulse amplitude maybe set to a safety margin greater than the pulse amplitude capturethreshold, and the pacing pulse width may be set to the test pulse widthused to determine the pulse amplitude capture threshold or to a maximumavailable pulse width. In one example, the LV pacing parameters set atblock 108 include setting the pulse amplitude to twice the capturethreshold amplitude and setting the pulse width to the maximum availablepulse width.

It is recognized that in some examples, if the capture thresholdcriteria are unmet at block 104, a low-voltage, single pulse pacingthreshold test may be repeated using a different pacing electrode vectorand/or after repositioning the extra-cardiovascular lead 16. Successfulcapture using the low-voltage, single pulse pacing output configurationmay be achieved when a different pacing electrode vector and/ordifferent lead position may enable.

If capture threshold criteria for the low-voltage, single pulse pacingoutput configuration cannot be satisfied at block 104, the controlmodule 80 starts a low-voltage, fused pacing threshold test at block110. The low-voltage fused pacing threshold test may be started bycontrolling LV therapy module 85 to deliver a test composite pacingpulse at a starting test pulse amplitude and starting composite pulsewidth. The starting composite pulse width is defined by the number ofindividual pulses delivered and their respective individual pulsewidths. For example, the starting test pulse amplitude may be at themaximum available pulse amplitude (e.g., 8 V) or a lower pulse amplitudethat is at least a safety margin less than the maximum available pulseamplitude of LV therapy module 85. The starting test composite pulsewidth may be the maximum composite pulse width available, e.g., 8 ms ifup to four consecutive fused pulses each having a 2.0 ms individualpulse width are delivered. In other examples, the maximum compositepulse width may be 10 ms, 12 ms or more. Alternatively, the startingcomposite pulse width may be less than the maximum composite pulse widthavailable, e.g., at least two individual, fused pulses of 2 ms each fora 4 ms composite pulse width. In some examples, the starting test pulseis delivered at a maximum amplitude and/or composite pulse width outputsetting available that still satisfies the capture threshold criteria.For example, the starting pulse amplitude may be an amplitude safetymargin less than the maximum available pulse amplitude, and/or thestarting composite pulse width may be a pulse number (or time interval)safety margin less than the maximum number of individual sequentialpulses (or maximum composite pulse width) producible by the LV therapymodule 85.

Safety margins may be defined for the pulse amplitude and/or for thepulse width for the low-voltage, single-pulse pacing outputconfiguration. Similarly, safety margins may be defined for the pulseamplitude and/or composite pulse width (or total number of individualpulses) for the low-voltage fused pacing output configuration. Thesepulse amplitude safety margins and pulse width safety margins may bedefined the same or uniquely for the low-voltage, single-pulse pacingoutput configuration and for the low-voltage fused pacing outputconfiguration. The pulse amplitude safety margins may be defined as afixed voltage or as a percentage of the pulse amplitude capturethreshold. The pulse width safety margins may be defined in millisecondsor as a percentage of the pulse width capture threshold or as a pulsenumber (of individual pulses each having an individual pulse width) inthe case of the low-voltage, fused pacing output configuration.

Capture of heart 26 by a test composite pacing pulse is detectedautomatically by control module 80, e.g., based on an R-wave sensed fromthe cardiac electrical signal received by sensing module 86 followingthe pacing pulse and/or based on other analysis of the cardiacelectrical signal received by sensing module 86. Alternatively, captureis verified by a user interacting with external device 40. If capture isverified after delivery of a composite pacing pulse that meets theacceptable capture threshold criteria, applied at block 112, controlmodule 80 enables the low-voltage, fused pacing output configuration atblock 114. Acceptable capture threshold criteria may include a maximumpulse amplitude capture threshold and/or maximum composite pulse widthcapture threshold. For example one or both of the pulse amplitudecapture threshold or the composite pulse width threshold may be requiredto be at least a respective safety margin less than the maximumavailable pulse amplitude and/or less than the maximum availablecomposite pulse width. To illustrate, the acceptable capture thresholdcriteria applied at block 112 may require the pulse amplitude capturethreshold be equal to or less than 50% of the maximum available pulseamplitude when the maximum composite pulse width is delivered. Forinstance if the maximum output for the low-voltage, fused pacing outputconfiguration is a composite pulse having an amplitude of 8 V andcomposite pulse width of 8 ms, the acceptable capture threshold criteriaapplied at block 112 may be a maximum pulse amplitude threshold of 4 Vwhen the total composite pulse width is 8 ms.

If the acceptable capture threshold criteria are met, control module 80enables the low-voltage, fused pacing output configuration at block 114.As described below, the low-voltage, fused pacing output configurationmay be enabled by control module 80 by enabling or activating switchesincluded in LV therapy module 85 that allow multiple LV holdingcapacitors to be sequentially discharged to a single output line acrossa selected pacing electrode vector.

The selected pacing electrode vector for delivering low-voltage, fusedpacing pulses may be the same as the pacing electrode vector that wouldbe used if the low-voltage, single pulse pacing output configuration hadbeen enabled. In other words, the two different low-voltage pacingoutput configurations may be controlled to use the same or differentpacing electrode vectors. The two low-voltage pacing outputconfigurations (and the high-voltage pacing output configuration) arenot defined by or limited to any particular pacing electrode vector. Asused herein, the “pacing output configuration” refers to how the pacingpulses are produced by the LV therapy module 85 or the HV therapy module83. The pacing output configuration is not defined by or limited to aparticular pacing electrode vector used to deliver pacing pulsesproduced by the selected pacing output configuration.

The low-voltage fused pacing control parameters are set by controlmodule 80 at block 116. These control parameters may include the pacingpulse amplitude and the composite pacing pulse width. The pacing pulseamplitude may be set a respective safety margin above the pulseamplitude capture threshold or to a maximum available pulse amplitude,e.g., 8 V. The composite pacing pulse width may be set to a respectivesafety margin longer than the pulse width capture threshold or to amaximum available composite pulse width or pulse number, e.g., 8 ms orfour individual pulses. The composite pacing pulse width may be set toat least twice the maximum pulse width available for the single pacingpulse with the composite pacing pulse including at least two fusedindividual pulses.

If the fused pacing threshold test performed at block 110 does notresult in the capture threshold criteria being satisfied at block 112,control module 80 may repeat the fused pacing threshold test using adifferent pacing electrode vector and/or after repositioning theextra-cardiovascular lead 16. Successful capture using the low-voltage,fused pacing output configuration may be achieved when a differentpacing electrode vector and/or different lead position is available. Ifthe fused pacing threshold test still does not result in the capturethreshold criteria being satisfied at block 112, control module 80enables a high-voltage pacing output configuration at block 120. Asdescribed below, enabling the high-voltage pacing output configurationby control module 80 may include setting a variable shunt resistance fordelivering at least a minimum electrical current to switches included inHV therapy module 83 to maintain desired switches in an active or closedstate during a pacing pulse.

A capture threshold test may be performed at block 122 to determineappropriate high-voltage pacing control parameters that are set at block124. The capture threshold test may be performed by controlling HVtherapy module 83 to deliver one or more pacing pulses and determiningwhether capture occurred, automatically by control module 80 or manuallyby a user as described previously herein.

The HV therapy module 83 may be configured to deliver pacing pulses inthe range of 10 V to 40 V, inclusive, in one example. The capturethreshold test may be initiated by delivering a test pacing pulse havinga pulse amplitude at or near the minimum pacing pulse amplitudeavailable from HV therapy module 83, e.g., 10 V. The test pulse may alsobe delivered at a relatively narrow or minimum available pacing pulsewidth. In one example, the starting test pulse delivered during thecapture threshold test at block 122 is a 10 V pulse having a 2 ms pulsewidth. If capture is achieved, the pacing control parameters for thehigh-voltage pacing output configuration are set at block 124.

In an illustrative example, the pacing control parameters set at block124 may include a pulse amplitude of 10 V and a pulse width of 10 mswhen the 10 V, 2 ms pulse successfully captures heart 26. The leadingedge voltage amplitude of the pacing pulse may cause pain or discomfortto the patient due to extra-cardiac capture of excitable tissue such asskeletal muscle. As such, the HV pacing control parameters may include apacing pulse amplitude set to the minimum pulse amplitude that capturesthe patient's heart at a relatively short test pulse width, e.g., 2 ms,and a pacing pulse width that is set at to a relatively large safetymargin greater than the test pulse width. The safety margin may be afixed interval, e.g., 6 ms, 8 ms, 10 ms or other predetermined interval,greater than the test pulse width. Alternatively, the safety margin maybe defined as a multiple of the test pulse width such as two times,three times, four times, five times, or other predetermined multiple. Amaximum pulse width limit may be defined in some examples. In oneexample, the capture threshold test is performed at a pulse width of 2ms, and a safety margin of 8 ms is added to the 2 ms test pulse width toset the high-voltage pacing pulse width at 10 ms. The leading edgevoltage of the HV pacing pulses is not increased above the capturethreshold amplitude during pacing, but the large pulse width safetymargin used in this case provides a high likelihood of successfullycapturing the heart.

If the starting test pulse does not capture the heart, control module 80may control the HV therapy module 83 to deliver test pacing pulses athigher voltage amplitudes and/or pulse widths. In one example, thethreshold test for the high-voltage, pacing output configurationincludes delivering pacing pulses having a pulse width of 2 ms at astarting pulse amplitude of 10 V and increasing the voltage until apulse amplitude capture threshold is identified up to a maximum of 40 Vfor the 2 ms pacing pulses.

The pacing control parameters set at block 124 include setting thepacing pulse amplitude to the pacing capture threshold voltage amplitudeand a pacing pulse width of 10 ms. In some cases, an amplitude safetymargin may be added to the pulse amplitude capture threshold to set thepacing pulse amplitude at block 124 in addition to setting the pacingpulse width to 10 ms, which may be a maximum available pacing pulsewidth for the high-voltage pacing output configuration. In otherexamples, longer pacing pulse widths may be available, but, for a givenpacing pulse amplitude, the delivered energy of a pacing pulse that islonger than 10 ms may not increase significantly due to the decay rateof the pacing pulse.

At block 126, control module 80 may enable top-off charging of a highvoltage holding capacitor included in HV therapy module 83 when thehigh-voltage pacing output configuration is selected based on thecapture threshold testing. When pacing pulses are delivered in thehigh-voltage pacing output configuration, the high voltage holdingcapacitor of HV therapy module 83 may be charged in anticipation of arequired pacing therapy. Prior to a first pacing pulse, the charge on ahigh voltage holding capacitor, e.g., capacitor 210 of FIG. 9, may betopped off to the programmed pacing pulse voltage amplitude. Forexample, top-off charging may be enabled up to one second prior todelivering a pacing pulse, or upon anticipating a need for delivering apacing pulse, which may be the first pacing pulse of a series of pulsesdelivered for capture threshold testing, ATP, post-shock pacing,bradycardia pacing or tachyarrhythmia induction. As such, HV therapymodule 83 may be enabled by control module 80 to perform capacitorcharge top-off charging at times that a shock therapy may not be neededand in response to selecting the high voltage pacing outputconfiguration. In this way, delivery of the pacing pulse energy from theHV therapy module is readily available upon scheduling a pacing pulse.

After enabling a selected pacing output configuration and correspondingpacing control parameters (at blocks 106 and 108 or at blocks 114 and116 or at blocks 120 and 124), ICD 14 delivers pacing pulses as neededaccording to programmed pacing therapies or tachyarrhythmia inductionprotocols at block 130. Bradycardia pacing pulses, ATP pulses, asystolepacing pulses post-shock or during atrioventricular conduction block,entrainment pulses prior to T-shock delivery for tachyarrhythmiainduction, burst pulses for tachyarrhythmia induction, or other pacingtherapies or tachyarrhythmia induction sequences may be deliveredaccording to programmed pacing control parameters, including timingintervals such as ventricular lower rate intervals, atrio-ventricularpacing intervals, ATP inter-pulse intervals, etc., using the selectedpacing output configuration.

It is contemplated that in some examples, the pacing parameters set atone of blocks 108, 116 or 124 may include enabling and/or disabling oneor more pacing therapies. For example, if the high-voltage pacing outputconfiguration is enabled with a relatively high pacing amplitude that istolerable for short intervals of time but intolerable by the patient forrelatively longer periods of time, ATP therapy may be enabled which isof relatively short duration. Asystole pacing post-shock or duringatrioventricular conduction block, which can be life-saving, may also(or alternatively) be enabled. Bradycardia pacing, which can bedelivered over extended periods of time, may be disabled. As such,setting HV pacing parameters at block 124 by control module 80 mayinclude enabling ATP therapy and/or asystole pacing and disablingbradycardia pacing. Asystole pacing may be provided when an R-wave isnot sensed for an asystole back up pacing interval, e.g., 1.5 to 2seconds. On the other hand, if the low-voltage, single pulse orlow-voltage fused pacing output configuration is enabled, bradycardiapacing and/or other pacing therapies that may extend over relativelylong periods of time (e.g., minutes or hours) may be enabled at block108 or 116, in addition to ATP and asystole pacing or other shortduration or life-saving pacing therapies if the patient is highlytolerant of the extra-cardiovascular pacing pulses delivered by LVtherapy module 85. If the selected low-voltage pacing outputconfiguration causes patient discomfort, bradycardia pacing may bedisabled at block 108 or 116 with ATP and/or asystole pacing therapyenabled.

It is recognized that in some patients reliable capture of heart 26 maystill not be achieved even using the HV pacing output configuration.Even if capture is achieved, a relatively high pacing capture thresholdmay exceed a tolerable level of pain caused by extra-cardiac capture ofsurrounding skeletal muscle. In this case, the HV pacing outputconfiguration may be disabled such that extra-cardiovascular pacingtherapies are not delivered by ICD 14. In some cases, re-positioning oflead 16 and/or selection of a different pacing electrode vector mayenable capture at a comfortable pacing output level.

Although FIG. 8 is described as including two low-voltage pacing outputconfigurations, the techniques described may be utilized with only asingle low-voltage pacing output configuration (e.g., only thelow-voltage single pulse pacing output configuration or only thelow-voltage fused pacing output configuration) or with more than twolow-voltage pacing output configurations. Likewise, there may beinstances in which the techniques can be used with more than onehigh-voltage pacing output configuration.

FIG. 9 is schematic diagram 200 of HV therapy module 83 coupled to aprocessor and HV therapy control module 230. HV therapy module 83includes a HV charging circuit 240 and a HV charge storage and outputmodule 202. Processor and HV therapy control module 230 may be includedin control module 80 for controlling HV charging circuit 240 and HVcharge storage and output module 202. HV charge storage and outputmodule 202 includes a HV capacitor 210 coupled to switching circuitry204 via a pulse width control switch 206 for coupling the HV capacitor210 to electrodes 24 a, 24 b and/or housing 15 to deliver a desired HVelectrical stimulation pulse to the patient's heart 26. HV capacitor 210is shown as a single capacitor, but it is recognized that a bank of twoor more capacitors or other energy storage devices may be used to storeenergy for producing electrical signals delivered to heart 26. In oneexample, HV capacitor 210 is a series of three capacitors having aneffective capacitance of 148 microfarads.

Switching circuitry 204 may be in the form of an H-bridge includingswitches 212 a-212 c and 214 a-214 c that are controlled by signals fromprocessor and HV control module 230. Switches 212 a-212 c and 214 a-214c may be implemented as silicon-controlled rectifiers (SCRs),insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductorfield-effect transistors (MOSFETs), and/or other switching circuitcomponents.

When control module 80 determines that delivery of an electricalstimulation pulse from HV therapy module 83 is needed, switchingcircuitry 204 is controlled by signals from processor and HV therapycontrol module 230 to electrically couple HV capacitor 210 to a therapydelivery vector to discharge capacitor 210 across the vector selectedfrom electrodes 24 a, 24 b and/or housing 15. The selected electrodes 24a, 24 b and/or housing 15 are coupled to HV capacitor 210 by opening(i.e., turning off or disabling) and closing (i.e., turning on orenabling) the appropriate switches of switching circuitry 204 to pass adesired electrical signal to the therapy delivery electrode vector. Theelectrical signal may be a monophasic, biphasic or other shaped CV/DFshock signal for terminating a ventricular tachyarrhythmia when VT or VFis detected.

For example, when a bi-phasic CV/DF shock is needed, one of switches 212a, 212 b and 212 c may be closed simultaneously with one of switches 214a, 214 b and 214 c without closing both of the “a,” “b” or “c” switchesacross a given electrode 24 a, 24 b or housing 15, respectively, at thesame time. To deliver a biphasic pulse using electrode 24 a and housing15, for instance, switch 212 a and 214 c may be closed to deliver afirst phase of the biphasic pulse. Switches 212 a and 214 c are openedafter the first phase, and switches 212 c and 214 a are closed todeliver the second phase of the biphasic pulse. Switches 212 b and 214 bremain open or disabled in this example with electrode 24 b not selectedor used in the therapy delivery vector. In other examples, electrode 24Bmay be included instead of electrode 24A or simultaneously activatedwith electrode 24A by closing switch 212 b during the first phase andclosing switch 214 b in the second phase of the illustrative biphasicpulse.

When control module 80 enables the high-voltage pacing outputconfiguration, capacitor 210 is charged to a programmed pacing pulseamplitude by HV charging circuit under the control of processor and HVtherapy control module 230. Switches 212 a-212 c and 214 a-214 c arecontrolled to be open or closed by processor and HV therapy controlmodule 230 at the appropriate times for delivering a monophasic,biphasic or other desired pacing pulse by discharging capacitor 210across the pacing load presented by heart 26 and a selected pacingelectrode vector. The capacitor 210 is coupled across the selectedpacing electrode vector for the programmed pacing pulse width.

In the example shown, the high-voltage pacing output configuration maybe enabled using electrodes 24 a and 24 b carried by lead 16. Housing 15may be unused by holding switches 212 c and 214 c open. Depending on theimplant location of ICD 14 and lead 16 and the resulting electricalstimulation delivery vector between the housing 15 and one or both ofelectrodes 24 a and 24 b, greater recruitment of skeletal muscle mayoccur when housing 15 is included in the pacing electrode vector. Alarger volume of skeletal muscle tissue may lie along a vector extendingbetween the distal portion 25 of lead 16 and housing 15 than along avector extending between the two electrodes 24 a and 24 b along leaddistal portion 25. In the example configurations of FIGS. 1A-2C, forexample, a pacing pulse may be delivered between the electrodes 24 a and24 b to limit skeletal muscle recruitment compared to a pacing electrodevector that includes housing 15. In other electrode configurations andimplant locations, the electrodes used to deliver extra-cardiovascularpacing pulses by HV therapy module 83 may be selected to provide adelivery vector that minimizes the volume of skeletal muscle included inthe pacing electrode vector while directing sufficient energy to theheart 26 for capturing and pacing the heart.

A biphasic pacing pulse, such as pulse 70 of FIG. 7, may be deliveredbetween electrodes 24 a and 24 b by producing a positive-going portionby closing switch 212 a and switch 214 b for a first portion 70 a ofpulse width 74 to discharge HV capacitor 210 across electrodes 24 a and24 b through heart 26. The switches 212 a and 214 b are opened, andswitches 212 b and 214 a are closed to deliver the negative-going phase,portion 70 b, of the biphasic pacing pulse 70. All switches of switchingcircuitry 204 are opened upon expiration of the pulse width 74, e g.,based on a time out of a pulse width timer included in processor and HVtherapy control module 230.

Between pacing pulses, as long as VT or VF are not being detected, theHV capacitor 210 is charged to the programmed pacing pulse amplitude. HVcharging circuit 240 receives a voltage regulated signal from powersource 98 (FIG. 5). HV charging circuit 240 includes a transformer 242to step up the battery voltage of power source 98 in order to achievecharging of capacitor 210 to a voltage that is much greater than thebattery voltage. Charging of capacitor 210 by HV charging circuit 240 isperformed under the control of processor and HV therapy control 230,which receives feedback signals from HV charge storage and output module202 to determine when capacitor 210 is charged to a programmed voltage.A charge completion signal is passed to HV charging circuit 240 toterminate charging by processor and HV therapy control module 230. Oneexample of a high voltage charging circuit and its operation isgenerally disclosed in U.S. Pat. No. 8,195,291 (Norton, et al.),incorporated herein by reference in its entirety. While not shown in theexample of FIG. 9, in other examples electrodes 28A, 28B and 30 may beselectively coupled to HV therapy module 83 via additional switchesincluded in switching circuitry 204 so that HV pacing pulses may bedelivered using a pacing electrode vector that includes electrodes 28A,28B and/or 30.

HV charge storage and output module 202 is shown to include a shuntresistance 250 in parallel to the pacing load shown schematically asheart 26 when electrodes 24A and 24B are selected as the anode andcathode (or cathode and anode, respectively) of the pacing electrodevector. It is recognized that a shunt resistance may be provided inparallel to the pacing load for any selected pacing electrode vector,for example shunt resistance 252 is shown schematically if the pacingelectrode vector includes electrode 24B and housing 15. Likewise a shuntresistance may be provided in parallel to the pacing load when thepacing electrode vector includes electrode 24A and housing 15.

Switches 212 a-212 c and switches 214 a-214 c may require a minimumcurrent flow to hold them closed (i.e., ON or enabled) for passingcurrent as capacitor 210 is discharged. This minimum current may be onthe order of approximately 10 milliamps. Depending on the pacing loadimpedance and other conditions, the electrical current passing throughenabled switches of switches 212 a-212 c and 214 a-214 c may fall belowthe minimum current required to keep the switches closed as capacitor210 is discharged across a selected pacing vector. If the currentpassing through a respective switch falls below the minimum currentrequired to keep the switch closed, the switch may open (or becomedisabled) causing premature truncation of the pacing pulse, which couldresult in loss of capture. As such, a minimum pacing pulse voltageamplitude may be set for the high-voltage pacing output configuration inorder to reduce the likelihood of the electrical current produced duringcapacitor discharge falling below the minimum current required tomaintain a stable state of enabled switches of switching circuitry 204during a programmed pacing pulse width.

The shunt resistance 250 or 252 may be a variable resistance that is setto match a pacing electrode vector impedance so that the load acrossheart 26 using a selected pacing electrode vector matches the shuntresistance. In this way, current through the switching circuitry 204 maybe maintained at or above a minimum current required to maintain astable state of enabled switches of switching circuity 204 during thepacing pulse. If the shunt resistance 250 is higher than the pacingelectrode vector impedance across heart 26, the electrical currentapplied to selected switches of switching circuitry 204 may fall belowthe minimum required to maintain the enabled state of the selectedswitches.

If the shunt resistance 250 or 252 is lower than the pacing electrodevector impedance, current produced by discharging capacitor 210 may beshunted away from the pacing load, e.g., the pacing electrode vectorbetween electrodes 24 a and 24 b and heart 26, resulting in less energydelivered to heart 26, which may result in loss of capture. Accordingly,processor and HV therapy control module 230 may be configured toretrieve a pacing electrode vector impedance measurement from impedancemeasurement module 90 and set the shunt resistance 250 (or 252) to matchthe pacing electrode vector impedance.

In other examples, a minimum voltage charge of capacitor 210 may be setto provide the minimum current required to maintain an enabled state ofselected switches of switching circuitry 204, but pacing energy may beintentionally shunted away from the pacing load including heart 26 inorder to reduce the delivered pacing pulse energy. If the pacingamplitude capture threshold is below the minimum voltage amplituderequired to maintain the minimum current to keep switches 212 a-212 cand 214 a-214 c on when they are enabled by processor and HV therapycontrol module 230, the energy delivered across the pacing electrodevector may be reduced by setting the variable shunt resistance 250 (or252) to a value that is less than the pacing electrode vector impedance.This current shunting may reduce skeletal muscle recruitment caused bythe extra-cardiovascular pacing pulse while still providing effectivecapture of heart 26.

Since the range of pacing load impedances and pacing voltage amplitudesmay vary between patients and at different times within a patient, avariable shunt resistance may be provided to enable selection of theappropriate resistance for shunting the required current through theswitching circuitry. It is contemplated, however, that in some examplesa fixed resistance shunt may be provided. For example, the resistanceneeded to shunt current to the switching circuit when the pacing loadimpedance is high may still shunt some current to the switchingcircuitry when the pacing load impedance is relatively lower. An optimalvalue for a fixed resistance shunt may be determined based on empiricaldata, e.g., typical pacing load impedances and pacing pulse voltageamplitudes used clinically.

FIG. 10 is a flow chart 400 of one method that may be performed by ICD14 as part of enabling a high-voltage, pacing output configuration atblock 120 of FIG. 8. At block 402, processor and HV therapy controlmodule 230 may control the HV therapy module 83 to apply electricalcurrent to switching circuitry 204. Current is applied to selected onesof switches 212 a-212 c and 214 a-214 c in order enable or activate theselected switches to select a desired pacing electrode vector. A minimumelectrical current is required in order to maintain an enabled switch ofswitching circuitry 204 in the ON or closed state. If the current is toolow, the switch may open.

At block 404, processor and HV therapy control module 230 may receive afeedback signal from switching circuitry 204 indicating that theselected switches are ON or enabled. If the selected switches ofswitching circuitry 204 are not enabled by the current applied at block402, the applied electrical current may be increased at block 405. Thefeedback signal may be a sampled electrical current signal or a sampledimpedance signal in switching circuitry 204.

Once the desired switches are enabled, an impedance measurement may bemade at block 406 by impedance measurement module 90 under the controlof processor and HV therapy control module 230. In some examples, thevariable shunt resistance 250 is adjusted to match the measured pacingelectrode vector impedance at block 410. By setting the variable shuntresistance 250 to match the pacing load resistance, the enabled switchesof switching circuitry 204 will remain enabled by the required minimumcurrent when capacitor 210 is discharging across the pacing load. Inanother example, the electrical current on an output line to the pacingload from HV charge storage and output module 202 is sampled. If thesampled electrical current is zero, the shunt resistance 250 isdecreased.

In other examples, the control module 80 may control high voltagetherapy module 83 to perform a pacing amplitude threshold test at block408. If the minimum charge voltage of capacitor 210 required to maintaina minimum electrical current applied to enable switches of switchingcircuitry 204 is greater than the pacing amplitude capture threshold,the variable shunt resistance 250 may be adjusted to a resistance thatis less than the pacing load impedance. A shunt resistance that is lowerthan the pacing load resistance will shunt current away from the pacingload and thereby reduce energy delivered across the pacing electrodevector to the patient's heart. For example, a minimum 10 V charge ofcapacitor 210 may be required in order to apply and maintain the minimumelectrical current needed to keep selected switches of switchingcircuitry 204 enabled. During a threshold test, if a pacing pulse havingthe minimum 10 V amplitude and a relatively short pulse width, e.g., 2ms, captures the patient's heart 26, the pulse amplitude capturethreshold may be less than 10 V and even more likely less than 10 V whena longer pulse width is used, e.g., 10 ms. A lower energy pacing pulsemay be tested by decreasing the variable shunt resistance so that somepacing energy is shunted across shunt resistor 250 rather than acrossthe pacing electrode vector. If capture still occurs, the pacing capturethreshold is less than the minimum voltage charge of capacitor 210 thatis required to produce the minimum current for enabling the switches 212a-212 c and 214 a-214 c of circuitry 204.

If this is the case, the variable shunt resistance 250 may be adjustedat block 410 to a resistance that is less than the pacing electrodevector impedance to reduce the energy delivered to heart 26 (andsurrounding skeletal muscle) when capacitor 210 is charged to theminimum voltage and discharged across the pacing load. If the pulseamplitude capture threshold is equal to or greater than the minimumcharge voltage of capacitor 210, the variable shunt resistance may beset to match the pacing electrode vector impedance. As such, thevariable shunt resistance 250 provided in parallel to the pacingelectrode vector may be adjusted by processor and HV control module 230based on the pacing electrode vector impedance (e.g., matching thepacing electrode vector impedance). In other examples, the variableshunt resistance 250 is set based on pacing electrode vector impedanceand the pulse amplitude capture threshold. When the pulse amplitudecapture threshold is equal to or greater than the minimum charge ofcapacitor 210 required to maintain a minimum required current toswitches of switching circuitry 204, the variable shunt resistance 250may be set to match the pacing electrode vector impedance. When thepulse amplitude capture threshold is less than the minimum requiredcharge of capacitor 210, the variable shunt resistance 250 may be set toa value that is less than the pacing electrode vector impedance.

In some examples, some or all of the process shown by flow chart 400including measuring impedance at block 406 and adjusting the variableshunt resistance based on the pacing electrode vector impedance at block410 is performed for every pacing pulse delivered by ICD 14 when thehigh-voltage, pacing output configuration is enabled. In this way, thevariable shunt resistance may be adjusted on a pulse-by-pulse basis tomatch (or in some cases be less than) the pacing load resistance forevery pacing pulse and thereby minimize the likelihood of any of theswitches of switching circuitry 204 being inadvertently disabled due tolow current flow, which could result in a non-delivered or prematurelytruncated pacing pulse and loss of capture.

FIG. 11 is a conceptual diagram of LV therapy module 85 according to oneexample. LV therapy module 85 may include a capacitor selection andcontrol module 504, a LV charging circuit 514, and a capacitor array610. Capacitor array 610 may include multiple holding capacitors 612,614, 616 and 618 that can each be charged by LV charging circuit 514 toa programmed pacing pulse amplitude. The holding capacitors 612, 614,616 and 618 are coupled to a respective output capacitor 632 a-632 d(collectively 632), 636, or 638 via respective switches 622, 624, 626,and 628 to deliver low-voltage pacing pulses. Each of holding capacitors612, 614, 616 and 618 has a capacitance that is less than the effectivecapacitance of high voltage capacitor 210 of HV therapy module 83. Forexample each of holding capacitors 612, 614, 616 and 618 may have acapacitance of up to 6 microfarads, up to 10 microfarads, up to 20microfarads or other selected capacitance, but all have a capacitancesignificantly less than the effective capacitance of high voltagecapacitor 210.

Power source 98 (FIG. 5) may provide regulated power to LV chargingcircuit 514. LV charging circuit 514 may be controlled by a statemachine in capacitor selection and control module 504 to charge all orselected holding capacitors 612, 614, 616 and 618 using a multiple ofthe battery voltage of power source 98, e.g., four times the batteryvoltage. LV charging circuit 514 charges capacitors 612, 614, 616 and/or618 as needed for delivering low voltage pacing pulses, either singlepacing pulses or composite pacing pulses, according to a selectedlow-voltage pacing output configuration.

In some examples, the LV therapy module 85 includes three pacingchannels 602, 604 and 606. Each pacing channel is capable of producing asingle pacing pulse when a respective holding capacitor 612, 616 or 618is discharged across an output capacitor 632, 636, or 638, respectively.Pacing channel 602 includes a back-up holding capacitor 614 that may beused for delivering back-up pacing pulses, e.g., when a low-voltage,single pulse pacing output configuration is selected. Back-up holdingcapacitor 614 may be used to deliver an individual pulse of a compositepacing pulse when the low-voltage, fused pacing output configuration isselected.

Depending on the number of extra-cardiovascular electrodes coupled toICD 14, one or more channels may include multiple selectable outputsignal lines. For example, channel 602 is shown in this example toinclude multiple selectable pacing output signal lines 642 a-642 d thatmay be selectively coupled to holding capacitor 612 and back-up holdingcapacitor 614 via closure of one or more of electrode selection switches634 a-634 d. For example, multiple electrodes carried by lead 16 may becoupled to pacing channel 602 and a pacing electrode vector may beselected from the multiple electrodes by closing certain ones ofswitches 634 a-634 d.

Pacing channels 604 and 606 are shown having single output signal lines646 and 648 that are coupled to respective holding capacitors 616 and618 via respective switches 626 and 628. In other examples, all threepacing channels 602, 604 and 606 may be provided with a single outputsignal line or with multiple output signal lines to enable selection ofa pacing electrode vector from among multiple extra-cardiovascularelectrodes coupled to ICD 14, e.g., any of electrodes 24A, 24B, 28A,28B, and 30 of lead 16 shown in FIGS. 1A-4.

When the low-voltage, single channel pacing output configuration isenabled by control module 80, any one of the pacing channels 602, 604and 606 may be used to deliver a single pacing pulse. The single pacingpulse may be delivered by discharging one of the holding capacitors 612,614, 616 or 618 across a selected pacing electrode vector via arespective output capacitor 632, 636 or 638 when a respective switch622, 624, 626 or 628 is closed. The output line 642 a, 642 b, 642 c, or642 d used to deliver pacing current from pacing channel 602 may beselected via a respective electrode selection switch 634 a-634 d. Theswitches 622, 624, 626 or 628 that enable discharge of a holdingcapacitor 612, 614, 616, or 618, respectively, may be enabled bycapacitor selection and control module 504 at the appropriate time whena pacing pulse is needed and maintained in an active, enabled stateuntil the single pacing pulse width is expired.

For example, pacing channel 602 may be coupled to pace/sense electrode28A, pacing channel 604 may be coupled to pace/sense electrode 28B andpacing channel 606 may be coupled to pace/sense electrode 30 in theexamples shown in FIGS. 1A-2C. If additional pace/sense electrodes areavailable, or if defibrillation electrodes 24A and 24B are also used forpacing, the additional electrodes or defibrillation electrodes 24A and24B may be coupled to a pacing output channel, such as channel 602, toprovide multiple selectable pacing electrode vectors.

When control module 80 selects the low-voltage, fused pacing outputconfiguration, the pacing channels 602, 604 and 606 are tied together byswitches 620 a-620 d and 630 to enable individual pulses to be deliveredacross a selected pacing electrode vector from a single output signalline 646. For example, control module 80 may enable the low-voltage,fused pacing output configuration by activating switches 620 a-620 b and630 to tie pacing output lines 642 a-642 d and pacing output line 648 topacing channel 604. Control module 80 controls capacitor selection andcontrol module 504 to enable pacing channel switches 622, 624, 626 and628 (and at least one electrode selection switch 634 a-634 d of pacingchannel 602) in a sequential manner to couple a respective holdingcapacitor 612, 614, 616 or 618 to output signal line 646 to deliver asequence of fused, individual pulses to produce a composite pacingpulse.

In various examples, depending on the particular pacing channel and leadand electrode configuration used with ICD 14, some electrode selectionswitches shown in FIG. 11 may not be required. Furthermore it isrecognized that less than four holding capacitors or more than fourholding capacitors may be included in a capacitor array 610 for use indelivering a sequence of fused pacing pulses when the low-voltage, fusedpacing output configuration is selected.

Capacitor selection and control module 504 selects which holdingcapacitors 612, 614, 616 and 618 are coupled to output line 646 and inwhat sequence by controlling respective switches 622, 624, 626 and 628.A sequence of pulses may be delivered to produce a composite pacingpulse by sequentially discharging holding capacitors 612, 614, 616 and618 one at a time (or one combination at a time) across a respectiveoutput capacitor 632, 636 and 638 by sequentially enabling or closingthe respective switches 622, 624, 626 and 628. For example, at least twoof holding capacitors 612, 614, 616 and 618 are sequentially dischargedto produce a composite pacing pulse produced by at least two fusedindividual pulses. Output line 646 may be electrically coupled to apacing cathode electrode carried by lead 16 and a return anode electrodecarried by lead 16 (or housing 15) may be coupled to ground. The pacingcathode electrode and return anode electrode may correspond toelectrodes 28A and 28B as shown in FIG. 3 or FIG. 4 in one example, orany pacing electrode vector selected from electrodes 24A, 24B, 28A, 28B,30 and/or housing 15 shown in FIGS. 1A through FIG. 4.

In some examples, a low-voltage, fused pacing pulse is delivered bydelivering an individual pulse from pacing channel 604 and 606sequentially followed by a third, longer individual pulse delivered bypacing channel 602 by discharging both capacitors 612 and 614simultaneously. The first two individual pulses may be 2.0 ms in pulsewidth and the third pulse may be 4.0 ms in pulse width for a compositepacing pulse width of 8 ms. The higher capacitance of the parallelcapacitors 612 and 614 allows for the third individual pulse to belonger in pulse width while maintaining a pulse amplitude thatsuccessfully captures the heart. All three individual pulses aredelivered via output line 646 because output configuration switches 620and 630 are enabled for the low-voltage fused pacing outputconfiguration.

In other examples, selected ones of holding capacitors 612, 614, 616,and 618 are discharged sequentially. For example, to deliver a compositepacing pulse, such as pulse 50 in FIG. 6, each of holding capacitors612, 614, 616 and 618 are discharged for 2.0 ms, one at a time in fusedsuccession to deliver a composite pacing pulse having a pulse width of 8ms. Each holding capacitor 612, 614, 616 and 618 is charged to thepacing pulse amplitude set for the low-voltage, fused pacing outputconfiguration, e.g., 8 V or less, which may be based on a fused pacingcapture threshold test and the maximum available pulse amplitude of theLV therapy module 85. Other examples of a LV therapy module andcomposite pacing pulse techniques that may be used in conjunction withthe techniques disclosed herein are generally disclosed in theabove-incorporated U.S. Pat. Application 62/262,412 and thecorresponding US. Pat. Application Publication No. 2017/0157399 (filedon the same day herewith).

FIG. 12 is a flow chart 700 of a method performed by ICD 14 forselecting one of the pacing output configurations described herein fordelivering extra-cardiovascular pacing pulses by the therapy deliverymodule 84. The method of flow chart 700 may be performed upon initialimplant of ICD 14, periodically thereafter, and/or upon determining aneed for pacing pulse delivery. At block 702, control module 80 selectsa pacing output configuration. Control module 80 selects the pacingoutput configuration from among at least a first low-voltage, pacingoutput configuration and a high-voltage pacing output configuration. Insome examples, control module 80 selects from a single-pulse pacingoutput configuration of the LV therapy module 85, a composite-pulsepacing output configuration of the LV therapy module 85 and ahigh-voltage pacing output configuration of the HV therapy module 83. Insome examples, control module 80 selects the pacing output configurationbased on a programmed selection, and in other examples control module 80actively selects the pacing output configuration based on testsperformed by ICD 14, e.g., based on capture threshold testing asdescribed in conjunction with FIG. 8 above.

If the selected pacing output configuration is not a low-voltage, pacingoutput configuration, “no” branch of block 704, control module 80controls the HV therapy module 83 to deliver one or moreextra-cardiovascular pacing pulses according to a pacing protocol atblock 710. HV therapy module 83 may be controlled to charge the HVcapacitor 210 (FIG. 9) to a pacing pulse amplitude via transformer 242and enable discharging of the HV capacitor 210 via switching circuitry204 to deliver the extra-cardiovascular pacing pulse(s). The switchesincluded in switching circuitry 204 may be maintained in an active statefor a pacing pulse width by setting a variable shunt resistance 250/252as described above in conjunction with FIG. 10. In some cases the HVcapacitor 210 is charged to a minimum voltage required to provide theelectrical current required to maintain selected switches of theswitching circuitry 204 in an active (closed) state for the duration ofthe pacing pulse width. In other examples, the HV capacitor 210 may becharged to a pacing pulse voltage amplitude that is greater than theminimum voltage required to provide current to maintain selectedswitches in an active state but less than the voltage required togenerate a defibrillation or cardioversion shock having a shock energythat meets the patient's defibrillation threshold.

If a low-voltage pacing output configuration is selected at block 704,and the selected configuration is a single-pulse pacing configuration,“yes” branch of block 706, the control module 80 controls the LV therapymodule 85 to deliver one or more single-pulse, extra-cardiovascularpacing pulses at block 708, according to a pacing protocol. Thesingle-pulse pacing pulses are delivered by charging one holdingcapacitor of the LV therapy module 85 to a pacing pulse voltageamplitude and discharging the holding capacitor for a pacing pulsewidth, which may be the maximum pacing pulse width available from the LVtherapy module 85, as described above in conjunction with FIG. 8.

If a low-voltage, pacing output configuration is selected, but not thesingle-pulse pacing configuration, “no” branch of block 706, controlmodule 80 controls the LV therapy module 85 to deliver composite pulsepacing at block 714. One or more composite pacing pulses, each includingtwo or more fused individual pulses delivered within the compositepacing pulse width, are delivered according to a pacing therapyprotocol. Each composite pacing pulse is delivered by charging at leasttwo different holding capacitors of LV therapy module 85 and dischargingthe at least two different holding capacitors on a common output linesequentially in time as described above in conjunction with FIG. 11. Insome examples, the control module 80 may be configured to select fromthe high-voltage pacing output configuration of HV therapy module 83 anda single low-voltage pacing output configuration, which may be eitherthe single-pulse pacing output configuration or the fused pacing outputconfiguration of LV therapy module 85.

ICD 14 is configured to select the pacing output configuration fromamong at least a first low-voltage pacing output configuration and ahigh-voltage, pacing output configuration to provideextra-cardiovascular pacing to a patient's heart in a manner thatconserves ICD battery longevity, minimizes patient discomfort, whilepromoting reliable capture of the patient's heart by delivered pacingpulses.

Thus, a method and apparatus for delivering cardiac pacing pulses usingan extra-cardiovascular ICD system have been presented in the foregoingdescription with reference to specific embodiments. In other examples,various methods described herein may include steps performed in adifferent order or combination than the illustrative examples shown anddescribed herein. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure and the following claims.

1. An extra-cardiovascular implantable cardioverter defibrillator (ICD)comprising: a high voltage therapy module including: a high voltagecapacitor having a first capacitance; a high voltage charging circuitconfigured to charge the high voltage capacitor; and first switchingcircuitry configured to couple the high voltage capacitor acrossextra-cardiovascular electrodes coupled to the ICD; a low voltagetherapy module including: a plurality of low voltage capacitors eachhaving up to a second capacitance that is less than the firstcapacitance; a low voltage charging circuit configured to charge the lowvoltage capacitors; and second switching circuitry configured toselectively couple the plurality of low voltage capacitors to theextra-cardiovascular electrodes coupled to the ICD; and a control modulecoupled to the high voltage therapy module and the low voltage therapymodule and configured to: select a pacing output configuration fromamong at least a low-voltage pacing output configuration of the lowvoltage therapy module and a high-voltage pacing output configuration ofthe high voltage therapy module; control a respective one of the lowvoltage therapy module and the high voltage therapy module to deliverextra-cardiovascular pacing pulses in the selected one of thelow-voltage pacing output configuration or the high-voltage pacingoutput configuration via the extra-cardiovascular electrodes.
 2. The ICDof claim 1, further comprising a sensing module configured to receive acardiac electrical signal via the extra-cardiovascular electrodes,wherein the control module is configured to select the pacing outputconfiguration by: controlling the low voltage therapy module to deliverat least one pacing pulse in a first low-voltage pacing outputconfiguration; determining from the cardiac electrical signal if the atleast one pacing pulse captures a patient's heart; determining if firstpacing capture threshold criteria are met in response to determining ifthe at least one pacing pulse captures the patient's heart; andselecting the pacing output configuration as the first low-voltagepacing output configuration in response to the first pacing capturethreshold criteria being met.
 3. The ICD of claim 2, wherein the controlmodule is configured to determine if the first capture thresholdcriteria are met by determining if at least one pacing pulse deliveredusing the first low-voltage pacing output configuration captures thepatient's heart when the at least one pacing pulse has a pulse amplitudethat is at least a safety margin less than a maximum pacing pulseamplitude producible by the low voltage therapy module.
 4. The ICD ofclaim 2, wherein the control module is further configured to select thepacing output configuration by selecting the high-voltage pacing outputconfiguration in response to determining that the first pacing capturethreshold criteria are not met.
 5. The ICD of claim 2, wherein thecontrol module is further configured to select the pacing outputconfiguration by: in response to determining that the first pacingcapture threshold criteria are not met, controlling the low voltagetherapy module to deliver at least one pacing pulse in a secondlow-voltage pacing output configuration of the low voltage therapymodule; determining from the cardiac electrical signal if the at leastone pacing pulse delivered in the second low-voltage pacing outputconfiguration captures the patient's heart; determining if second pacingcapture threshold criteria are met in response to determining that theat least one pacing pulse delivered in the second low-voltage pacingoutput configuration captures the patient's heart; and selecting thepacing output configuration as the second low-voltage pacing outputconfiguration in response to the second pacing capture thresholdcriteria being met and the first pacing capture threshold criteria beingunmet.
 6. The ICD of claim 5, wherein controlling the low voltagetherapy module to deliver extra-cardiac pacing pulses in the firstlow-voltage pacing output configuration comprises enabling the lowvoltage therapy module to deliver a single-pulse pacing pulse having apulse width up to a maximum pulse width, and wherein controlling the lowvoltage therapy module to deliver extra-cardiovascular pacing pulses inthe second low-voltage pacing output configuration comprises enablingthe low voltage therapy module to deliver a composite pacing pulsecomprising at least two fused individual pulses produced by dischargingat least two different capacitors of the plurality of low voltagecapacitors in sequence, wherein the composite pacing pulse has a pulsewidth that is longer than the maximum pulse width of the single-pulsepacing pulse.
 7. The ICD of claim 5, wherein the control module isconfigured to determine if the second capture threshold criteria are metby determining from the cardiac electrical signal if at least onecomposite pacing pulse delivered using the second low-voltage pacingoutput configuration captures the patient's heart when the at least onecomposite pacing pulse has a pulse amplitude that is at least a safetymargin less than a maximum pacing pulse amplitude producible by the lowvoltage therapy module and has a composite pacing pulse width comprisingat least two fused individual pulses.
 8. The ICD of claim 5, wherein thecontrol module is further configured to select the pacing outputconfiguration by selecting the high-voltage pacing output configurationin response to determining that the second pacing capture thresholdcriteria are not met.
 9. The ICD of claim 1, wherein the control moduleis further configured to set a plurality of pacing control parametersfor the selected pacing output configuration when the selected pacingoutput configuration is the low-voltage pacing output configuration by:determining a pacing amplitude capture threshold for the low-voltagepacing output configuration; setting a pacing pulse amplitude by addinga safety margin to the pacing amplitude capture threshold; and setting apulse width to a maximum pulse width available for a single-pulse pacingpulse.
 10. The ICD of claim 1, wherein the control module is furtherconfigured to set a composite pacing pulse width to include at least twofused individual pacing pulses when the selected pacing outputconfiguration is the low-voltage pacing output configuration.
 11. TheICD of claim 1, wherein: the high voltage therapy module comprises avariable shunt resistance; the control module is configured to controlthe high voltage therapy module to deliver the extra-cardiovascularpacing pulses in the high voltage pacing output configuration when thehigh voltage pacing output configuration is selected by: applying anelectrical current to enable the first switching circuitry; determiningan impedance of a pacing electrode vector selected from theextra-cardiovascular electrodes; and setting the variable shuntresistance in parallel to a pacing load of the pacing electrode vectorbased on the impedance.
 12. The ICD of claim 11, wherein the controlmodule is configured to set the variable shunt resistance equal to theimpedance.
 13. The ICD of claim 11, wherein the control module isfurther configured to: control the high voltage therapy module toperform a test to determine a pacing amplitude capture threshold;determine if the pacing amplitude capture threshold is less than aminimum voltage charge of the high voltage capacitor required to producethe electrical current to enable the first switching circuitry; and setthe variable shunt resistance to be less than the impedance in responseto the pacing amplitude capture threshold being less than the minimumvoltage charge.
 14. The ICD of claim 1, wherein the control module isfurther configured to enable a first pacing therapy comprisingextra-cardiovascular pacing pulses delivered using the selected pacingoutput configuration and disabling a second pacing therapy comprisingextra-cardiovascular pacing pulses from being delivered using theselected pacing output configuration.
 15. The ICD of claim 14, whereinthe first pacing therapy comprises at least one of anti-tachycardiapacing (ATP) therapy and/or asystole pacing therapy and the secondpacing therapy comprises bradycardia pacing therapy.
 16. The ICD ofclaim 1, wherein the control module is further configured to enabletop-off charging of the high voltage capacitor in response to selectingthe high voltage pacing output configuration.
 17. The ICD of claim 1,wherein; the low voltage charging circuit is configured to charge theplurality of low voltage capacitors up to a first maximum voltage; andthe high voltage therapy module is configured to charge the high voltagecapacitor up to a second maximum voltage that is greater than the firstmaximum voltage.
 18. The ICD of claim 17, wherein: the high voltagecharging circuit comprises a transformer for charging the high voltagecapacitor up to the second maximum voltage; and the low voltage therapycircuit is configured to charge the plurality of low voltage capacitorsup to the first maximum voltage without a transformer.
 19. A methodperformed by an extra-cardiovascular implantable cardioverterdefibrillator (ICD) having a low voltage therapy module and a highvoltage therapy module, the method comprising: selecting by a controlmodule of the ICD a pacing output configuration from among at least alow-voltage, pacing output configuration of the low voltage therapymodule and a high-voltage, pacing output configuration of the highvoltage therapy module, the high voltage therapy module comprising ahigh voltage capacitor having a first capacitance and the low voltagetherapy module comprising a plurality of low voltage capacitors eachhaving up to a second capacitance that is less than the firstcapacitance; and controlling a respective one of the low voltage therapymodule and the high voltage therapy module to deliverextra-cardiovascular pacing pulses in the selected one of thelow-voltage pacing output configuration and the high-voltage pacingoutput configuration via a plurality of extra-cardiovascular electrodescoupled to the ICD.
 20. The method of claim 19, wherein selecting thepacing output configuration comprises: controlling the low voltagetherapy module to deliver at least one pacing pulse in a firstlow-voltage pacing output configuration; determining if the at least onepacing pulse captures a patient's heart from a cardiac electrical signalreceived by a sensing module of the ICD via the extra-cardiovascularelectrodes; determining if first pacing capture threshold criteria aremet in response to determining if the at least one pacing pulse capturesthe patient's heart; and selecting the pacing output configuration asthe first low-voltage pacing output configuration in response to thefirst pacing capture threshold criteria being met.